Magic level is sort of like a skill already, but in your case it is saved in the players table instead of the player_skills table.
Also, a new skill is no problemo friendo to create, however you will have issues using a standard Cipsoft client to view the new skill as a skill-bar. I suggest you use OTC, then you can easier add a new skill-bar into the client (I guess this is part of your plan?)
Now to answer your question: look at how the other skills has been added in the source code, copy pasta the shizzle out of one of 'em and success will come your way. Also not that there are some iterators loading the skills in different ways, and it might look something like
for (int i=0; i < 6, ++i)
Which means if you would add Another skill, you would have to change that 6 to a 7 since you will be having 7 'skills'