how do I create a database that uploads to an existing one

I have a main database that is located on an office server. Six people
connect to it using a replicated copy of a front end, each of which are
linked to the same backend.

We travel frequently to locations where data line speed keeps us from
entering data directly into our main DB. What I would like to do is create a
portable version that can append the data from off site locations to the main
file. I just keep getting stuck. Suggestions?