On-demand process of one entity which creates record of another entity. Want to associate created record with a record of a third entity.

As the title says, I have a workflow in Dynamics CRM 2016 which is on a Complaint entity with a lookup to a Contact record. In this workflow, I create a Case record related to the Complaint entity. The Case form has a subgrid for Contacts (N:N) and should take the Contact from the Complaint lookup and add it to this subgrid. I don't see any option to do this within the process itself. I would try to use the msdyncrmWorkflowTools Associate Entity step but I think this would only allow me to associate a record with the Complaint record which is not what I want. Does anyone have any suggestions?

To my knowledge, the only way to implement this would be to create a plugin. I would appreciate any other suggestions or even some information as to how to implement such a plugin.

Thank you!