MIDI Interface programming

Hi,

i need some help in programming MIDI-Interfaces. i want to program something like an arpeggiator, which transformes the midi noteOn-signals of my keyboard (accords) to a melody or a rythm. A software synthesizer, which is controlled by my program, plays these new midi-notes.

My problem is to programm an interface between the Input Port of the computer and an other application. I know how to open the midi In port and how to open a midi out port. But the application (sw synthesizer) i want to control provides no own out-port, that i could open. do I have to program an own MIDI IN-Port? And how can i do it?

Please help me, if you understood the problem:-)
Guido