Ok, checked this out. Apparently the only way to add or edit a tail number on an existing repaint will be to go through the regular repaint process.
This linked tutorial outlines that process. It's not really 'easy' but will get the job done.
Repainting in FS2004
EDIT: Oh BTW, this tutorial is for FS9. You didn't mention which version of FS you are using, so if it's FSX there may be other steps involved which aren't covered in this tutorial.