You don't need external modem: you need only
LIONSTRACS MEDIASTATION X76.

This keyboard is open surce and is working everytime with VST (NI KONTAKT, AKAI VST, a lot of sinths e.t.c.) what you want that you get. You will have in next to years new wish and than you will only change VST and it is work as new expander.

This is future.