MEP2018 Plus - 17.0.1.141
Apparently, saving a transition proposes the following folder:
C:\ProgramData\MAGIX\Movie Edit Pro Plus\2018\Plugins\TransitionPresets
The reply from Magix was to navigate to this folder using the Import tab and load it from there.
I thought that in the previous version of MEP, this showed up under Fades somewhere when saving to the above subfolder, but I see that it doesn't. Your own transitions have to be put somewhere else, like under:
C:\ProgramData\MAGIX\Movie Edit Pro Plus\2018\Plugins\MoveTransitions
and they will show up under Templates/Transitions/Transitions Basic.
On the French side, Panagix has produced a method to personalize MEP by adding in tiles, amongst other things. Take a look at the pdf.
https://www.magix.info/us/tutorials/personnalisation-de-vido-deluxe--1197612/
If there is interest, I will make an attempt to translate this to English, but only the pdf, not the install files that he includes.
My only worry is that a new version comes out and supersedes or wipes this work.
As for saving own effects, I haven't looked at this yet. At least for Titles, Magix added in Templates/Title templates/My Own with the latest patch.
John CB