As a Power? I'd say giving a flat +2 to a particular skill or a Skill Substitution for an entire skill sounds about right. The second is more like what you want, it could result in something like this:
Wide Experience [-1]:
You have extensive experience, and by extensive we mean not hundreds, but likely thousands of years. Due to this extensive experience, you may use your Scholarship instead of another skill for all uses of said skill. This power may only be taken for intellectual skills, or ones that can be learned purely intellectually, physical abilities require constant practice. You may take this Power multiple times, but no more than your Scholarship rating.
Then take it the maximum of 5 times for Discipline, Lore, Empathy, Deceit, and Survival (or something like that). Combined with a Superb Scholarship, this'll make you a fairly badass character. The only other skills needed by your typical immortal are Weapons, Athletics, and Resources (though Endurance, Presence, and Conviction all seem reasonable to have at Good).