Yes, you can change the default Shapefile used for the Location Map. First, you will need to put your Shapefile in the $IMAGINE_HOME/etc directory. Then you will need to specify the name of your Shapefile with the IMAGINE Preference Editor.
IMAGINE Preference Editor > Applications > Map Composer > Default Location Map file
It is recommended that you also review the other preferences for the Location Map in the Map Composer category of the IMAGINE Preference Editor.