Custom module "add" for related lists

Custom module "add" for related lists

Our business has two types of products that are currently in the same module. I am interested in transferring all of one of those types of products to a different module so that each module can contain only the fields that relate to that type of product. So I would be splitting "Products" into "Devices" and "Phone Services" This part I can manage.

What I need help with is how I can have both of these modules interact with our "Members" module so that we can "add" multiple "Devices" or "Phone Services" from the related lists in the "Members" module without "creating new." However the only module that seems capable of this is the default "Products" module. Any custom module I create does not allow me to add in the related list section, I can only create new.


Help!

Thank you