How to make case for a controller?

Hi guys, I have slowly been teaching myself solidworks here and there over the years. I am just getting back into it after a couple years.

I have a quick question on how to approach something.

I am trying to make a protective case for my Virtual Reality controllers. I was able to find a .step file of a model of the controller already made. I thought i could just use that model and offset entities to build the case, but turns out i cant edit the step file. It keeps giving me feature errors when trying to convert it.

Is there any other way to approach this? any other suggestions? I have attached the model to this post.

SolidworksModeling And Assemblies