What is weird about what you said is I have over the past couple of weeks imported stp
files into Solidworks with some being assemblies and they came in as individual parts within
the assembly without a hitch. I just did one prior to reponding to you.
So, unless I am missing something and not understanding what you are asking, you should
be able to import a stp file and get individual parts within your assembly.
If you are able to contact the vendor and see how he is saving the step file. Maybe he needs to change the
options settings on his end.
What Options are you setting under the Options button when you do your File > Open > .step?
You are in luck.
File>Open>Change file type to STEP. Click on Options... Under General, there is an option called "Import multiple bodies as parts".
That should get you where you need to go in one step. One step.