Well, looking at the BK-7m's page on Roland's site I see nothing about its capabilities to work with iPad or anything like it. Maybe I didn't understand what exactly you plan to do with it.

The DGX indeed can only connect through USB, perhaps you'll need to use a computer or some sophisticated device in order to connect it with something else.

A module like this can be fun to use with some small MIDI keyboard, although today a lot of them don't have this type of MIDI output either.