Fillin Forge has no API for registering your own field types.
The types are listed inside the plugin (FORMFORGE_Field_Types::get_types()) and there is no filter for the list. The only hook that draws a field is formforge/render_field_body, which Fillin Forge Pro uses for its own types. See Field Type Hooks.
To collect something the built-in types do not cover, combine them: a Hidden field, an HTML Block, a Calculation field (Pro) or a Repeater (Pro).