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.