populate sister files

I have a volunteer file, and associated with it and with the same primary key value, are AdditionalInterests, Education, and other files. When I write out a record to the Volunteers file, I want to write records with the same primary key value out to the sister files, so that when the user has time to enter the extra information, the placeholder (primary key) is already there, and enforces the relational nature of the system.

what's the best way to write to the other few files when I write out a volunteers record?