Sounds like you have existing drives you want to put in the QNAP and access as standalone.
Short answer, Yes.
Longer answer, any data I deem as critical will get a good backup before making a change like this. You just trust the QNAP to not wipe out your data, but plan for the worse.
I would buy a drive of equal size that's on the QNAP enclosure approved list, put it in the QNAP and configure the way you want, then copy the data to the new QNAP device.