Adding a subset of Custom Fields per Package

Hi,

We are currently using custom fields so that our users can pre-map fields in documents, so they can be filled in later by our system automatically. We currently have a field for "Purchaser Name", "Purchaser Address", "Purchaser Phone Number", etc.

The issue is that our system supports up to 12 purchasers, and we don't want to show 3 custom fields for all 12 purchasers in the designer for every single transaction. For example, if there is only one purchaser in our system, then I want to be able to create the package and tell it to only use the 3 purchaser fields for a single purchaser.

Is this possible?

Note: In the API docs for the POST package endpoint, I see a "userCustomFields" property under roles > signers, however nothing happened upon trying to use this.