I don't know FOnline-code, but probably it's possible.
When you use FA on someone it's an action, it returns HP healed to target, XP to you by HP/3, minutes to your FA-CD depending on your FA skill. So, also you can create any real number which will be increased by using FA skill, and this real number will affect your total FA skill.
So like:
Action - use FA skill / OnUse FA / etc. depends on code or whatever it is
Condition - / if / etc.
Action - / <script command> / etc.
And then to everything what already there, add variable called Skill_FA_Progress or something, and after each use of skill it'll be increased, also it can be a formula with TaggedSkillMult and UntaggedSkillMult, also depending on value, than more Skill_FA_Progress, than lower number will be added (harder to train). And then: your FA skill = Skill_FA_Progress/100 or something.
So, for 50% FA skill you need real(50000) Skill_FA_Progres, by every use of FA skill you getting X of this number depending on tagged it or not and how high this real already.
I making triggers/scripts only at Oblivion and Warcraft, so don't know much about coding or something, but anyway it's almost similar logic commands. Warcraft and TES are two totally different kinds of trigger/script, but it works almost similar.