Hi Samantha,
I invite you to consider our Excel Updater solution which will allow you to efficiently add custom forms to multiple documents, and (in a second pass) optionally update the data, too.
For background, Custom data on Documents are treated differently than on data (such as Projects, Tasks, Issues, etc.), and one of the main differences is that they cannot be bulk attached; which is why we (recently) enhanced Excel Updater to fill that need.
Regards,
Doug