From what I'm gathering, you can disassemble other people's Zygarde and add them to your collection of cells.
I can only really confirm this with three random people on the internet, but no one is denying it, so I guess that answers your question. If anyone else would like to test, you can comment about your outcome below. That being said, if you trade away the Zygarde you developed from the cells scattered around Alola, those cells do not respawn - they're gone forever.
It's also possible to get more than 100 cells, however it won't say that you have more than 100. It will just say 100/100 until you go below 100.
Hope I helped. :)