Adding fields from custom module into template

Adding fields from custom module into template

How do I add a field or fields from a Custom Module into an email or letter template?  When I create a template the custom module does not show up, but if I set a workflow automation up, I can choose fields and data from any custom module.

Is this an oversight from the development team?

It seems crazy that you would allow us to create custom modules but don't let us do something simple like create a letter template from them.

Is this being worked on? did I miss something? or can it be done? thanks.