Yes, you can configure the HD500 footswitches to send midi messages. Check the MIDI ASSIGN page in the HD500. You can also use HD500 Edit to do this.

See manual Appendix B:

http://line6.com/data/6/0a06434dc1a55085c5d6532bf/application/pdf/POD%20HD500%20Advanced%20Guide%20v2.10%20-%20English%20%28%20Rev%20A%20%29.pdf

The trick is to think of it as a two part thing

1)   Assign a controller (footswitch or expression pedal) to toggle an effect or (for expression pedals) to change a parameter.

2)   Use a Midi CC to control the controller.

As the gents above have said, full details in the Appendix to the Advanced guide.  Make sure that your firmware is 2.0 or greater.