You might want to take a look at my "Main Campaign Modding" thread stickied at the top of this forum. I have an example of making a skill and getting it into a skillbook. I didn't use the Excel file though; I created the skilldata.txt for the skill directly in that example.

It also might be worth checking out these threads of mine where I had made custom skills:
Skill Case Study: http://www.larian.com/forums/ubbthreads.php?ubb=showflat&Number=527247

Summon Lords of Magic mod: http://www.larian.com/forums/ubbthreads.php?ubb=showflat&Number=523851

Making a preset class takes a bit more work than editing the skillsets of pre-existing presets like I had done in the Main Campaign Modding thread. I can't quite recall the steps to do so, but there is an xml file I believe that holds the attributes of the preset classes in the Data/Mods/ portion of a mod.