If you want to import it via MIDI as a single kit then you need to create a Percussion Map for it, in the same way as described in this thread: How might I import midi to transcribe drum parts? The percussion map defines the relationship between MIDI notes and the individual percussion instruments they represent. You need to specify the percussion map for the MIDI track.
I think if you have a project that contains a player for your new custom drum kit (as well as the new Percussion Map) then you can import the file into the current project (rather than a new project) and route it to that player.