You can create the macro in a utility database, then reference that database within the VBA IDE. First create the database containing the original macro and save it. Then, in a database where you want to use the macro, go to the VBA IDE (alt-F11), select Tools, References from the menu, browse to your new utility database and select Open to bring it back to the References dialog. It should be checked in the dialog, just click OK to return to the IDE.
Once it is referenced, you can just call its procedures and functions from your referencing database via VBA, in queries, reports or forms.