Yes. RAW Power can only open the system library and only the system library *CAN* be synched to iCloud (but it doesn’t have to be).
This is how you can do it:
1. Open your 100GB library in Photos.
2. Go to preferences and pick “Use as System Photo Library”. Photos will warn you that it will turn off iCloud Synching. That’s good – you want it to do that.
3. You can then open that library in RAW Power.
4. When you’re done, you can open the other library in Photos, tell Photos to use it as the System Photo Library, but then ENABLE synching.
I can imagine this might give you pause. In that case, I suggest you run a test by creating a tiny library with one or two unique images (ones not found in your iCloud Photo Library) and following the above steps. If the tiny library isn’t merged into your iCloud Photo library, then you know that these steps will work.