Moving custom field data to CustomPress field

On a project I started entering data into custom fields, but then I started using CustomPress.

There is still one meta key that contains information on about 50 posts and was not created by CustomPress; I would like to move the data from there into a CustomPress field to simplify the UI for my clients.

What is the best method to do this?
PS: I'm not afraid of custom PHP or SQL queries.