You can use any control that sends midi to act as a mod wheel, and you'll want to double check these things:
1. The control must send MIDI CC1 or be mapped at the concert level with the “Send to All > Your Keyboard Destination > CC 1 Modulation”
2. The control must be on MIDI Channel 1 for CC1 data to pass into MainStage’s plugins. Especially check this if you are using TouchOSC as it tends to send on different MIDI channels.
3. The Extra Section Song Section notes are not controlled by MIDI CC 1, rather the physical mod wheel itself is assigned to each of those extra section p-text boxes. So, if you are using a different control you will need to assign that control in the Layout mode to each of the extra section p-text boxes.
4. TouchOSC works like any other midi controller, it just sends MIDI wirelessly- but the assigning process is the same.
You can also learn how to program another fader/knob/expression pedal as your mod wheel in this video tutorial here: