Migrate support for flag

Okay, I need some directions on how to import some old data (non Drupal) into a new Drupal site.

People can create a user on the site and add (Flag) things to what we call their collection. This is simple a node with a "Add to Collection" flag button. Now, people was able to do this on the old site as well. I need a way to import that data into the new Drupal site.

What I can create is ex. a XML/CSV file with:
The Drupal "User id" and the "Node id" - As in this USER ID has this NODE ID in his collection.

Is it possible to import this? Perhaps using Feeds? I could write directly into the flag_content table but do I dare that?

Let me know your ideas and thanks for a great module and community.