I think that, unless the game/app. has a specific midi option, it isn't possible.

OPL music uses FM synthesis while MIDI uses a different approach (I think by using real instruments recorded samples).

OPL has nothing to do with MIDI (other than the fact that some games convert MIDI data to OPL commands on the fly), and when DOSBox gets OPL data from a game it's not in any format that resembles MIDI in any way. As a result, DOSBox has no ability to convert or redirect OPL data to anything other than a file containing raw OPL commands.