I think your odds of working are poor.
If you were successful you would be running 8gb in dual channel mode and the odd 8gb in single channel mode.
The Z170-P D3 is one of the odd motherboards that will support ddr3.
https://www.asus.com/us/Motherboards/Z170-P-D3/HelpDesk_QVL/
If you have a 16 gb stick, I suppose you could try it.
But, I see no 16gb sticks on the QVL list.
http://dlcdnet.asus.com/pub/ASUS/mb/LGA1151/Z170M-E_D3/Z170_DDR3_4Dimm_memory_QVL_report161222.pdf
If you want guaranteed working, buy a single kit of the size you need that is supported.
Either from the QVL list or from a ram vendor ram selection app for your motherboard.
Then, you could try your old ram to see if you can get any extra, but do not count on it...