Since having (eg.) two arms is a result of evolution, how can we, considering modern technology, force embryos to grow another limb?
[Polymelia](https://en.wikipedia.org/wiki/Polymelia) is known to me, but it is considered a birth defect.
I know that evolution is only possible because of random mutation.
I guess an alternative evolution needs parents to derive genes from. 

Therefore my question would be: 

Is it possible today, or would it be possible in the future to
trigger growth of limbs by forced Mutation and if so, how many generations would it take to come to the point, where the third limb is as efficient as the other two?