Trying to be helpful, but at the same time, trying to inspire you to do a bit of digging and research and understand what is needed. That is a very large part of custom watercooling - figuring out all the pieces and how it works the way you want it to work. I realize that this is 'an AIO' but at the same time, it really isn't because it is designed to be expandable and an entry-level segue into custom watercooling.
There are a couple ways of doing this, but both will require disconnecting tubing and fittings and draining the coolant - also will likely require you to refill with additional coolant/new coolant or top off with distilled water. If you get new coolant, try to flush the old coolant with tap water in the event there are any chances the 2 coolants are chemically incompatible, but most glycol based coolants are pretty inert.
Option 1-
The only issue with getting the 2-piece quick disconnect tubing would be that you'd really have to disconnect the tubing on the existing setup, and run both sets on a single run, while using both pieces of this set on the remaining side.
https://www.amazon.com/Alphacool-Eisbaer-Extension-Straight-Connectors/dp/B01MFC1QWP/ref=sr_1_18?tag=georiot-us-default-20&dchild=1&keywords=alphacool tubing&qid=1612550271&sr=8-18&ascsubtag=tomshardware-us-1197667786771355100-20&geniuslink=true
Meaning, take the part of tubing that does not have the disconnects and remove it, connecting it with a female/female butt connector to the end of the pieces with the quick disconnects, and then reconnecting back up. The Amazon 2- piece disconnect would be connected as one long piece and form the other side of the loop between the pump and radiator.
So, you'd also need something like this for the butt connector:
https://www.amazon.com/BXQINLENX-Computer-Cooling-Through-straight/dp/B01DXMELMY/ref=sr_1_8?dchild=1&keywords=water+cooling+g1/4+butt+connector&qid=1612671629&sr=8-8
This would go on the G1/4 fittings on the original AIO disconnect piece and the original non-disconnect length to make a single, long leg. Use the 'new 2-disconnect pieces, connect as 1 large piece, that is your other side.
Option 2 -
Replacing all the tubing with new fittings and tubing. Tubing sizes are measured in BOTH inside and outside diameter (ID/OD) and either in metric or imperial measurements. If you need to convert, use an online metric/standard conversion calculator or the calculator length conversion feature in your Operating System.
Determine tubing size. For an AIO, 3/8" (10mm) ID is probably just fine....no need to try and run large 1/2" (13mm) ID tubing.
Depending on inside diameter, the fitting will need to fit that, if using a barb, you just need the ID to match the tubing ID. So, 3/8" (10mm) ID tubing will fit a 3/8" (10mm) ID barb. Just need a hose clamp or zip tie to secure.
For compression fittings, you need to not only know the ID of the tubing but also the outside diameter (OD) so you know what size the collar needs to be to secure the outside of the tubing once screwed on. This matters, so match the ID and OD to the ID/OD sizes for the compression fittings you like.
Refilling the cooler -
Either option means draining and refilling the cooler once you are done. Given that the pump and reservoir are both in the same housing, you are likely better off mounting the radiator (assuming all tubing is connected) and leaving the pump disconnected from the CPU mount. A power strip with a toggle switch makes the next part much simpler.
Then, you will need to jump the 24-pin ATX power on the PSU by disconnecting from the motherboard and using a jumper tool
https://www.amazon.com/s?k=psu+jumper&ref=nb_sb_noss_2 or by using a small piece of wire or paper clip to jump green to black. If your PSU does not have multi-colored wiring, your safest bet is a jumper tool so you don't short the power supply and possibly electrocute yourself in the process.
With the power definitely turned off - fill the pump reservoir until it is full. Do not screw the fill cap on. Flip the power strip toggle to kick on the PSU, powering the pump. Let the pump push the coolant through the lines, but turn it off as soon as it gets low and before it runs dry. Refill the reservoir and repeat these steps multiple times. To help with the priming process, you can get a fill bottle and slowly fill the reservoir while the pump is running if you are so inclined.
https://www.amazon.com/1000ml-Bottl...rds=coolant+fill+bottle&qid=1612672438&sr=8-5
Continue this fill/cycle/stop/fill process until the cooler is as full as you can make it. Cap the fill plug and cycle the power back on. Rotate the case/radiator to dislodge air pockets and keep the pump higher than the radiator to allow air to collect, if possible. Kill the power, fill with coolant to displace any air, recap. Should be good to mount the pump back to the CPU, remove the ATX jumper and connect the 24-pin ATX back to motherboard.
Start up the system and make sure everything is going smoothly.
If you can do all these steps above, you can technically build, prime and fill a custom watercooling loop - the steps are exactly the same, just slightly different depending on what is in the cooling loop.