Hi Heather,
This is not really a “pre-sales” question, since the PTA Volunteer Sign-Up Sheets plugin is free. Assuming that’s the plugin you are talking about. =)
To answer your question: No (sorry), it’s not possible the way the plugin is coded right now. That would require adding another field to the database table and another field to the add/edit sheet form, and all the necessary programming to create and save those values, and then the programming to do the necessary checking to make all of that work (not sending emails when that field is set, etc.), plus some kind of extra functions to download the data and/or send out a custom email to those people at the end.
If that’s something you absolutely need, then you could hire me, or any other programmer (the code is open source), to add the needed functionality for you.
Otherwise, you could put your idea in the Feature Requests section of this forum for consideration for future updates.