Satellite as a sub-assembly?

[Fadamor]

I'm wanting to launch a 3-satellite set in a single launch vehicle. I have a suitable satellite saved. I have a suitable platform figured out and saved. What I can't figure out is how to have 3 of the satellites and the platform in the VAB at the same time. It won't let me save the satellite as a sub-assembly because there's a command module (the Probodobodyne HECS) on the satellite. Is there a way to save a build that includes a command module as a sub-assembly?

EDIT: I hate it when I find the answer shortly after asking the question.  I changed the root to the stack separator and it now looks like I successfully saved the satellite as a sub-assembly.

Edited September 30, 2019 by Fadamor

[Pecan]

Hoorah and congratulations!  You have indeed found the secret to saving sub-assemblies, which is making sure there's a free node where you want to attach them.  That usually means making the relevant separator / docking port or other part the root.