Is it within Shenron's or Porunga's capabilities to perform such a task? Since they couldn't even turn the Androids back into humans, I don't think they can.
I think so. Shenlong can't do stuff to people beyond his own power, that's why he can't turn 17 and 18 back into normal people, at least not without their consent. Transforming someone who asked into a Saiyan isn't a big deal.