Not sure what you asking for here..
If you want the D-50 to be non-velocity sensitive just edit the patches not to respond to velocity.
This will have no effect on the MIDI data sent by the D-50. Only the patches played by the D-50.
Some sequencer packages will allow you to set a fixed velocity value.

BTW - a 'real' non-velocity sensitive keyboard sets the velocity value at 64. Quite low..

So nice to see a message NOT in the general arranger keyboard forum. The rest of the forums seem very under used.
I'm sure some of the messages in the general arranger keyboard forums could be placed in other sections.