Use stp files in assembly

Hello,

is it possible to use STEP files as they are in assemblies? I know I can import them, but it seems they are converted into prt files, which is not what I want. (Background: We are using multiple CAD systems and I would like to have the standard parts in a neutral format so they can be reused for all CAD systems from our PLM and we don't need to check in the same standard parts multiple times).