How to create custom transitions in effects panel ?

Hi to all of you !

I am looking to make custom transitions in Premiere. I am well aware of the "Save Preset" function but I am looking to create my own transitions in the effects panel.

I know it is possible as some companies sell packages like these (see in my picture)

If any of you has the solution, you will make my searching efforts irrelevant but I will be so glad.

Cheers.