How can I create from my custom drum note to a pitch? (for fransposing)

[Bernard]

How can I create from my custom drum note to a pitch?

Or, strongly related, how can I transpose from a custom drum note to a 
standard piano note (pitch)?

I created a custom drummode for Djembe with custom notes. This displays 
fine.

Following is a snippet to create my custum "dba" note :

#(define djembe '(
         (dbass            default   #f           -2)
...
         )

drumPitchNames.dbass      = #'dbass
drumPitchNames.dba        = #'dbass  % db seems to be in use
...


The possible custom notes are :

one = \drummode { dba4 do ds r dbm dom dsm }

I have a soundfount for Djembe. The midi file contains the following 
value to sound Ok :

onemidi = { g4 a b r  fis gis ais r  }

So voice "one" displays the correct note in layout mode. And "onemidi" 
the correct midi sounds are heard.

Now I want to transpose "one" to "onemidi"

When I try to do so with :

  \transpose dba g { dba4 do ds r dbm dom dsm }

I get the error message :

 /tmp/frescobaldi-ejoKxg/tmp8WsKbH/transitie.ly:78:18: error: wrong 
type for argument 1.  Expecting pitch, found "dba"
  \transpose dba
                 g { dba4 do ds r dbm dom dsm }

So how can I solve this?