I see what you mean.
Yes of course, adding the TMG extension to your SPL on the basis of a valid TMG Class Rating would be utterly sensible.
What objection could there possibly be?
(I think OC619's process was conversion to a Part-FCL licence from a UK one.
By contrast, operating exclusively under Part-FCL is a different matter)
Whatever, do give them a call and please let us know how you get on.