I think of two ways of making this : 1- The game would make the parts (parts that you already have) you want to change make them look like robotic ones and having a better performance in those parts.(ej. You could robotize your arm , so it would look like a robotic one and it would be stronger) 2-You would enter the creature editor and there would be a new section of robotic parts.You could take off your organic parts and put new robotic ones and having better performance .
Well if we look at it scientifically. In no place in nature have we studied "Bio-Mechanics" Stuff like.. Wheels. You don't see animals rolling around on biological wheels. So then it wouldn't really make sense to evolve those bionic parts. Further more. There are only two reasons I see a race would want or need robotic limbs. 1. They somehow lost them. 2. They went all Iron Hands (+1 Internet to anyone who gets the reference) and decided the flesh is weak and begun to systematically replace their limbs. Either way. The way this could be accomplished in game could be, you create a limb in the tech editor which I assume would have a setting for creating limbs (Kinda like the difference in KSP between choosing to make a rocket or a space plane). After the limbs are created you can then select them from the menu in the organism editor.