Object importing position and tranform of a bone during animation

So, I have the following situation. There is one armature with no mesh on it, which I will use as a base for a customizable mech. There are also the mech parts, which are unrigged models. Is it possible to parent this unrigged model to one of the armature bones and make it follow its position/transform during animations? If so, how, and if not, why? Also, in case of such thing not being possible, what could be the best workaround for such case, taking optimization in mind, of course. And lastly, an extra: Can I make so one Armature plays two different animations at the same time? Such as, for example, lower body doing walk animation while upper body does attack animation?

A good approach to gaining knowledge and solving problems is to perform general searches before posting a question that may have already been asked and answered.

The Unity forum has numerous threads discussing rigs and attaching meshes to them for customizable characters.

There is also UMA which is a free system that is setup to perform this exact thing, plus a host of other options like full body and facial morphs, etc etc.

For your animation question - search for animation layers in the documentation. Quick answer - yes.