I got the idea for the muscle mimic from the TV show Hereos (if you haven't seen it yet you should). In the show there is a girl that can instantly learn any physical skill that she observed being done. I was thinking for Shadowrun you would have an adept power called Muscle Mimic with a rating. If you witness a physical skill being performed you can do a check (rolling the number of dice equal to your adept power rating) and gain that physical skill at the number of success you through (max being the number of success thrown by the observed person performing the skill). If that skill is used before the end of the game session then it becomes a permanent skill. Else the skill is lost. There would only be 3 levels of the power and they would cost the following:
Level 1 = 1 point
Level 2 = 4 points
Level 3 = 9 points
No skill could be improved with this power just learned initially. Any skill that is retained could be increased as per a normal skill.