I have a Process set up which creates a new entity (X) based on information from the original entity (Y). Most of the fields are mapped no problem (e.g. the Sign-up date in Y copies over into the Sign-up date field in X), but I can't figure out how I can copy data in a subgrid. Both have the same subgrid set up in the main form but I can't see where you could map between the two. 

The subgrid is a Lookup based on an N:N relationship, and it is on both forms for X and Y. 

When I go to create the Process, the grid just shows the following message:

The list of related records is not available when you create a process. To create a related record from a process, add a Create Record step. To update a related record, add an Update Record step.

I'm not really sure what this means? If I add an Update Record step, I just end up at the exact same page with the same message.

Any help on this would be greatly appreciated!