I filled my 4 drive nas with drives and because I don't yet have so much data that I need three drives, I set up a 2 drive parity. Once I need that space for data, can I easily move one of the parity drives into the pool and use it for data?

If you mean move it to the array, then yes. Simply perform a "new config" and relocate the parity drive to a new slot in the array. To move it to a pool, it depends on what filesystem you use in the pool and how many drives you have in the pool already.

If you mean the array, you can just stop the array, unassign parity2, start array, stop array, now reassign it as a data disk and start the array