You can. It has 2 slots, and 1 is in use.
Increasing RAM will not increase performance. If your library is large enough to blow out RAM, Roon will crash, not slow down. There is no “swap” on disk.
No, you should make sure your supply is at least 65w (you will need about 3.42amps at 19v), but be careful, after-market power supplies are the #1 reason why Nucleus units stop functioning.