That depends on exactly what you mean.
If you mean "could I add 'must find a suitable tutor/mentor in order to learn this' as an additional prerequisite for certain talents?", then that's not unreasonable, especially for specialised abilities such as martial arts or Force techniques.
If you mean "could a character gain additional talents beyond those granted by level progression by receiving training in them?", then that's definitely not covered in the rules, and could potentially be quite unbalancing. Personally, I wouldn't recommend it.