Currently, adding a TMG extension to an SPL/LAPL(S) needs the specified training to be done within an ATO.
EASA noticed that the ATO requirements are not suitable for SPL/LAPL(S) training and so has been working on an alternative. There will probably be an NPA, with a better set of regulations, published in the New Year.
I don't know of any ATO offering the training.
Meanwhile, glider pilots wanting to fly TMGs have the same option as has existed for years: NPPL SLMG.
Thanks for the answer. Maybe that wasn't clear enough, but I did the training for the TMG class rating with an ATO and passed the skills test. I was able to add the TMG class rating to my PPL, but I wasn't able to add it to my SPL, for the reasons stated.
Thus I'm wondering whether it's now possible to add the initial TMG class rating after passing the skills test to an CAA-issuede EASA SPL.