It depends a bit. The 200 reserved skills/spells are divided in 130 spells, and 70 skills.
Expanding the skills-list is as simple as upping the MAX_SKILLS number (as long as you use ascii pfiles, but most people do).
Expanding the spells-list is somewhat harder. The skill numbers match messages in lib/misc/messages. So of you wish to move the skills, remember to move change the message numbers as well. Also, changing MAX_SPELLS means doing something about the existing player and object files (scrolls/staves).