You cannot have custom fields in a sign up sheet at this time. But this is something we can certainly look into for future design.
A database might be able to better server what you are looking for in the meantime.
Thanks!
Ron
Ron Raley
Firehouse Solutions
P.O. Box 911
Leonardtown, MD 20650
Office: 301-475-1900
Email: Ron@FirehouseSolutions.com