At this time there are no add-ons for additional fields or to allow people to select multiple tasks at once.
Please note, though, that if you require your volunteers to have a WordPress user account, and they are logged in when they sign up, then the information will be automatically filled in on the form when they sign up, if that info already exists in their user account. If the info does not exist, then after they sign up for the first task, that info is saved with their user account and will be pre-populated in the fields for any future sign ups.
I have done a couple of very large custom programming jobs for other organizations using the free plugin as the core, and those have included custom fields and lots of other custom functionality. So, if there is something you absolutely need that isn’t already covered by one of the available extensions, you can always contact me privately to discuss programming a custom extension to meet your needs.