Writing back MIME repository files: Is it possible to write information back to a file that is stored in the MIME repository?

GuiXT doesn't offer a function to change or create MIME repository files.

You can either use SAP function modules (please look at the standard function SKWF_* in the function module library SE37). 
Or an easier way is to use the WebDAV interface to the MIME repository. See SAP documentation for details.