anything is possible...
Anything can be done; however, you risk when you do. Depending on how much and what you want to do, you risk changing the way the cue plays and you risk severely degrading the value of an older cue.
If you want to add inlays or ring work it's going to depend on many factors. What type of joint it is, do you want to add ring work at the handle or at the butt/joint...
What types of woods the cue is made of also determines the amount of risk...
You wouldn't want to try and have a cuemaker add ivory diamonds to snakewood points for instance.
Adding MOP dots to gabon ebony points isn't that difficult with a good refinish though...
Jaden