Yamaha DTXpress/DTXplorer/DTXtreme

Hi from a newbie

I have had my DTXpress for a number of years now and want to 
experiment a little further.  I want to be able to continue using teh 
DTX module for drums but simultaneous use the GM voices (e.g Strings)
to be triggered from an external MIDI keyboard (whose internal voices 
aren't so good).  I know it can be done and I have searched high and 
low for instructions (including this board) but I can't find any.  
Could anybody point me in teh right direction please.

Thanks

Carl