Can I "merge" disjoint parts?

I have a sketch that has a bunch of disjoint elements (rectangles mostly) lovingly arranged relative to one another. I'd like to end up with three parts (required, desired and optional) where each is made up of N sketch elements that may or may be contiguous. I'd like to then be able to add these to a diagram and manipulate their appearance etc. I've tried a bunch of things but I the parts don't seem to merge.

• Selected all the "desired" faces and extrude. That give me M parts, one for each contiguous set of sketch elements
• Union those extruded parts but they don't intersect so nothing happens
• Create one extrusion as "new" and then extrude the rest of the faces for that set as "add" with the extrusion part as the scope. Still had a bunch of parts.

Naively I want to merge a set of extrusion parts but they seem to have to be contiguous. Alternatively, grouping the parts together so that they can be added and manipulated together in a diagram would be fine. 

I see that I could create all the individual parts and then assemble them with connectors but that seems like a lot of work and since they are not contiguous, I'd have to redo all the offsets manually.

It may be that I'm just trying to do something that OnShape is not designed for but I'm hoping I'm just missing something. Thanks in advance.

Jeff