sure, but why? in the parent child relationship model, it's ok if you don't have any children at first (or ever) for some of the records. Based on what I know of your question at the moment, it doesn't make sense to create essentially empty records to "hold a place" for potential future child records. The sub table containing the child records will have it's own unique primary key, and will happily accept the foreign key (the parent) at any time.
When you go back to add child records later, you open the form back up, select the volunteer and then start adding interests, etc.
In fact, if your child table is one-to-many (multiple interests per volunteer, for example), the primary key of the child has to be unique (and independent of the parent key) anyway.
Am I missing something in your question?