Good question, Paolo. The manual doesn't quite take us, explicity, as far as how to load factory or user MixPresets into 'alien' families of Presets. But it is possible - with no need for diving into Sublime and getting busy with PITA clerical editing.
A method I've previously used for loading my own or a factory MixPreset into a Preset of an 'alien family' - though not exactly slick - goes something like this:-
1. While any Preset is loaded, hop over to the main MixerPreset browser and load a MixPreset that you intend to use in some other Preset family in which this MixPreset is not listed - i.e. is 'alien'.
2. In the Mix tab, open the "Preset:" popup menu above the Reverb strip at the far right of the tab, and in that menu select "Save settings to disk". In the window that opens, name it and save it.
3. Load a Preset from another 'family'. Go to the same popup menu as in 2 and select "Load settings from disk." Select and load your saved MixPreset from the window that opens.
4. When you've loaded one of your previously saved MixPresets in this way into an 'alien family' Preset, you won't see its name anywhere; you just have to remember that you have an 'alien' MixPreset loaded.
5. Depending on how 'alien' your saved MixPreset is to the Preset instrument of your choosing, you may find that your MixPreset changes the mixer configuration to something inappropriate for the loaded instrument Preset (e.g. your saved factory MixPreset from SyDimStrings can load 8 mixer channels for a Clarinet, lol.) Nevertheless, from what I've tried thus far, whatever happens in this cross-matching - no matter how seemingly chimerical - turns out to be logical and correct where it really counts. (Nicely done, VSL!)
6. In the right-hand Reverb strip, using the button marked with a floppy disk icon ("add current mixer settings as preset") will show your saved preset mirrored in the Presets browser only for the 'home' case; it's not listed in alien family Preset lists.
I'll leave you to explore the almost endless possible what-ifs.
(Currently I don't use any particular MixPresets in my template. I have every Synchron Player's Convo and Algo verbs switched off and all instruments centred, because I do all my spatialisation outside the Synchron Players. Same goes for all my Kontakts, other sample players and synths).