There are a couple ways you could choose to do this:
1) If it is for one particular volunteer, you can cancel the volunteer's sub request, then re-request as the administrator and send emails. To do this open the Live schedule from the Schedules pane inside of MSP, find the volunteer at the date on the schedule where they requested a swap, then right click and choose "Clear current flag". Then, right click again and choose "Flag as swap requested" and the program will ask if you want to re-send emails (Note you must be using the latest version for this to work).
2) If this is for multiple volunteers, you can send a general email from the Announcements pane using the Swap Requested Positions token in the token menu under "Potential Fill-in Positions." This will list all open swaps and send to people who are available to fill them.
3) If this is a consistent need, you can activate the "Auto Available Positions" email that is sent once a week to all volunteers who are available for any open swap requested positions or unfilled positions. To do this, go to the Web Terminal pane, click on "Web Terminal settings...", then click on Auto Reminders. You will then see an option to turn this on.