Hi Andre. You would not use post custom fields because those are for the Post content type.
Do not use Post fields; use regular fields to collect the data you need, then you map those fields to the user profile (user meta custom fields) in the Feed.
To store information in the user’s profile in custom fields, you use the User Registration “Create User” (or Update User) feed. You can see how to set that up here:
If you have any other questions, please let us know.