Form possibilities: link an entity to a relation field

Hello!

I am organizing short-shifts system, and for that I’ve created a database called shifts.

I wanted people to subscribe to the shifts through a form… So I have 168 shifts (entities) on my database. And I wanted to know if it’s possible not to create a new entity through a form, but to add and information on a pre-existing entity. For example, each shift entity has a field related to my volunteer workers database. So I wanted to add people on this field through the form…

Is that possible?

Thank you!