You can set up a channel strip in MainStage to have sustain automatically applied to the sound even if you don't have a sustain pedal by using the Midi Modifier.
You'll want to add Midi Modifier to your channel strip and change it to:
Input Event: Velocity
Re-Assign to: 64 Sustain
Then, move the bottom slider called "Add" to any number higher than 63.
Then, you'll be all set!