Adding Custom Module to Flex Relate Field

Hi All,

I just created a custom module and would like to include sub-panels for Notes, Calls, and Meetings. I don't like using the standard One To Many relationship because I don't want the relating field on the Notes, Calls, and Meetings records.

Ideally, I would like for the custom module to appear as an option in the Flex Relate dropdown on the Notes, Calls, and Meetings modules while at the same time having a sub-panel for each of those modules on the custom module records. However, I can't find a way to add the custom module as an option in the flex-relate dropdown. I have a feeling that may be a hard-coded feature that only includes out-of-the-box modules, but thought it was worth asking here.

Thanks in advance for your help!