I also think this is a good idea. Hireable professionals for your group which can perform some traits far better than your character. Different levels for these experts (poor, average, good, excellent, most famous) AND much higher cost than normal mercenaries would make sure that it's balanced and will not lead to powergaming.
I think these specialists should be NPC-like characters, so they would cost you 1 party slot each. These specialists shouldn't be scripted NPC characters, though: they just work for you and give a bonus to one skill, they are not companions like Marnid or Borcha. The locations and skills of these specialists should vary. You could find them from taverns, wilderness, as prisoners... lots of possibilities. These experts would be good at what they do (tracking, surgeon, trade etc.) but lousy on other skills (another balancing feature). And they are expensive.
And they should fight also, of course. Just imagine your frustration when Dark Hunter party slaugthers your tracker expert in an important battle. Until you find a replacement for your departed specialist you have to rely on the next best thing (which means you or one of your NPC's).
It wouldn't be an easy job to balance this, but if a good compromise between the weekly cost of a specialist (only high level characters should be able to afford specialists), their skill level and the appearance frerequency is found, it will not be too much of a powergaming feature but a rich add to the game. At least that's what I believe.