when reading Your question as written and understanding it how it is written, You are trying to import a STEP assembly into a single part.
This will result in at least one design body - maybe consisting out of several assembly components with no manifold contac - and as it sounds several other bodies, for what ever reason.
So the basic question more or less is concerning a mutlybody part.
And witihn a multibody part ONLY one body can be the current active body!
You can change them from one to another and back but You only can have one active body anyway!
So there is no function and even no program intend to acitviate all bodies witihn a single part.
Is this Your question?
Yes that's correct.
Basically, I have a motor step file (lots of parts) which I do not want to put in Teamcenter. I want to save it as a single part, thus 1 part number into Teamcenter.
The issue I have is the inactive parts are transparent.
How can I save this step file as a single part that looks solid?
there is an option during STEP import to merge bodies into one, as long as there is no Non-Manifold-Situation within those bodies.
With ST10 there even is an option to meld such bodies too and also to remove all internal holes for merging it into one single solid body
Maybe you can do an insert part copy of that multi-body part and select merge solid bodies? Perhaps with the "Adjust non-manifold conditions and merge" plus "Remove voids".
@Dazzie, I saw that other thread and was going to post it here but since you were on that thread as well there was little point. It doesn't completely solve the problem for a model I have but it's better.
The inactive solid body face style opacity is set by the adjustable setting accessed in Solid Edge options→View tab. If the option dim surrounding assemblies when in place activated is off, then the inactive body is shown in full opacity. If the option is on, then the inactive body is shown dimmed by percentage controlled by the slider.