Not sure what you mean . . . An accidental is just a note that isn't in the key signature. So sure, go ahead and add some. It really depends on what kind of music you're playing. If you're playing blues, accidentals won't matter at all, because blues songs don't have definite key signatures. If you're playing jazz, everyone will think you were really cool to add those dissonant notes. Anything else, and you might want to stick to key.