My understanding of OP's proposal is that the case comes with two fans, and there are two places to locate them. (That ignores the front panel and the AIO rad being mounted there.) One of those obviously mounts in the REAR panel near the top. The question seems to be whether or not to mount the last fan in the EXISTING opening in the TOP panel near the rear. That does not require any new holes, large or small.
Here's what I gathered from the OP:
So, H510 Elite. We know that one is preinstalled with 2 140mm fans at the front. The chassis supports up to 140mm(top) and 120mm only at the rear.
They have a Kraken AIO. The size is not specified, but from the following statement:
hey i wanted to ask that there were
2 fans that came with my nzxt elite, and
similar two fans came with my kraken aio cooler. now
i have attached one of the cooler fans on top of the nzxt elite case
the other one is useless though.
It must've been one of the 280mm models, because the rear doesn't support 140mm fans.
Combined with the inserted google drive pic, I came to the conclusion:
OP has a Kraken 280mm radiator installed up front. They did not remove the preinstalled 140mm chassis fans.
They took the fans from the 280mm AIO and installed one at the top. The 2nd one is useless because the rear doesn't support that size.
So their idea is to take that 2nd fan and instead install it in the empty space between the radiator and the 140mm fan that was installed at the top.
The panel is solid right there. There's no ventilation.
It's not like the idea isn't doable, it's just not practical without some DIY elbow grease.