Saving a file for later assemby

Hi,

I want to model geometry of a complicated tool for later studies on Multibody dynamics module.
As an start, I have created each part in different files and I have saved them as the mph file.
However, when I want to import them for the final assembly, They can not be imported since it seems that only files with the mphbin format can be imported.
Does anyone know how I can solve this problem? Thanks for the help.

Regards
Yashar