In Epo you can add additional source sites for the ePO server to look at when updating master repository.
In configuration> server settings> source sites. You can "edit" and add a new FTP site.
I havent actually tried this myself but beleive it should do the trick.
Note, You will also need to create an additional server task for a "repository pull" and point it to look at your own ftp source site. As the default task will only look elsewhere on failure.
Hope this makes sense, Please let me know if it works.