It's possible, but I think more complicated than necessary to have a single field that can point to one of multiple tables. You won't be able to use the automatic relationships if you do.
Maybe have 3 fields? 1 field records the type of handler, 1 field has a value if the handler is a user (ie, the username) and 1 field has a value if the handler is a team (ie: team name/id).
Your model can then have 1 relationship between the model and user tables, and 1 relationship between the model and team table. Which one you access depends on the value of the field that records the type of handler.
To make your life a little easier, you could write custom getHandler() and setHandler() methods that encapsulate the necessary logic.