I know this question is ancient.  But, I decided to answer it anyway.

I'm not really sure what it is you are asking.  If you can model a shirt, you can model a button and even a zipper.  

The final answer to your question is simple: You model them.

You can do that using several techniques.

One way to do that would be to simply box model them as separate objects.  A button is a flattened cylinder, a ziper can be much more complex.  You could also extrude them from the base mesh and then alter them accordingly.  You could use spline models and operations to model complex accessories with unique shapes.  Once you get one button made, clone the rest of them and move them into place.

It all depends on how detailed you want to get and what that portion of the model is meant to do.  If it's just made to sit there and look good, then just model it separately and "weld" it in place.  If it has to open, like a zipper, then you're dealing with morphs so you'll have to plan that accordingly as well.  You'll need to create a morphed (changed) version of the object and then use that as the morphed vertex data for whatever application you're using.

A hi-res zipper that opens/closes just like one in the real world is tough to do.  But, you can do a simple one fairly easily.  There are several tutorials on modelling zippers and, IIRC, there are a couple dealing specifically with Hexagon.  But, most of them would be applicable to working with Hex even if they weren't Hex-Specific.  I watch all sorts of tutorials and end up picking up good modelling practices and workarounds I can use in Hexagon.

Hexagon is very easy to use and you should have no trouble modeling any buttons, zippers and accessories you need if you can already model a basic shirt.