You can save any assembly file as a multi-body part. This is especially useful if you want to simplify imported assemblies or reduce the overhead of sub-assemblies which have been downloaded from other sources. It is also useful if you are going to send your assembly to another designer who doesn't need to change the assembly.
To save an assembly as a part, simply go to file, save as, and select .sldprt as the file type. You will have an option to save all components to the part, just exterior components (bodies), or only exterior faces (surfaces). The saved part will have no association back to the original assembly.