FWIW, as a full blown sequencer, Real Band DOES support MIDI... and keyboards and strings are two things that MIDI does very well.

If you want to add those you can do the following:

1) from the style picker, select a style that has keyboard and string in a style that matches your song

2) right click on a midi track and from the floating menu, pick "Generate MIDI track"

3) pick the instrument you want to add (Strings or keyboard)