Hello can you please clarify what you mean by combine or merge parts/files?
Simplify3D allows you to print multiple model files in the same build. If you're asking for the ability to merge two different overlapping meshes, no that's not currently a feature. But I'd be happy to document it as a feature request if that is your desired functionality.
"A bird does not sing because it has an answer. It sings because it has a song."