Yes it's possible to use every STM-Mixer in XTC-Mode by a simple trick:
1. Make a copy of your STM ....dev in your devices-folder and rename it to STM ....-XTC.mdl
2. Load this new ...mdl into your XTCProject.pro.
3. The Mixer-Window has to be open when saving the project. You better save it with the biggest windows-size (for example 24-channel view) because some Mixer-models have window resize problems.
4. Close SFP and open your Sequencer in XTC-Mode.
You will get an error-message that the preset-list is not 100% compatible....
Klick yes. After saving the preset-list new this message disappears.
That's it
5. You can use everything in the mixers and make presets for your projects.
You can even do routing-issues.
Limitation: In my case optimaster did not load as an insert. After trying it, i had to restart the program. All other plugins that i used worked fine as inserts and sends in the mixers.
Workaround: First load optimaster as XTC instrument in Cubase, then you can load it in the STM-mixer too.
Another tip
the dll-copy-method for XTC-instruments or effects provokes some confusion concerning the location where the host should find the specific devices.
1. Delete (or rename) your DevicesXTC-folder, it's really not necessary.
2. Open SCOPEVST.CFG and change the paths to the sfp devices folder
It should for example look like:
HostSynth C:/SFP/Devices/Sampler/VolksZampler.dev
ScopeStereo C:/SFP/Devices/Effects/Stereo/Reverb/*.*
ScopeMono C:/SFP/Devices/Effects/Mono/Others/*.*
ScopeStereo C:/SFP/Devices/Effects/Stereo/Others/*.*
ScopeMono C:/SFP/Devices/Effects/Mono/Modulation/*.*
ScopeStereo C:/SFP/Devices/Effects/Stereo/Modulation/*.*
ScopeMono C:/SFP/Devices/Effects/Mono/Filter/*.*
ScopeStereo C:/SFP/Devices/Effects/Stereo/Filter/*.*
ScopeMono C:/SFP/Devices/Effects/Mono/Dynamic/*.*
ScopeStereo C:/SFP/Devices/Effects/Stereo/Dynamic/*.*
ScopeMono C:/SFP/Devices/Effects/Mono/Distortion/*.*
ScopeStereo C:/SFP/Devices/Effects/Stereo/Distortion/*.*
ScopeMono C:/SFP/Devices/Effects/Mono/Delay/*.*
ScopeStereo C:/SFP/Devices/Effects/Stereo/Delay/*.*
ScopeMono C:/SFP/Devices/Effects/Mono/*.*
ScopeStereo C:/SFP/Devices/Effects/Stereo/*.*
ScopeSynth C:/SFP/Devices/Synths/*.*
ScopeMono C:/SFP/Devices/Plug-Ins/*.*
ScopeStereo C:/SFP/Devices/Plug-Ins/*.*
ScopeSynth C:/SFP/Devices/Plug-Ins/*.*
ScopeSynth C:/SFP/Devices/Modular III/*.*
ScopeSynth C:/SFP/Devices/Modular III/Patches/*.*
If you really want to use Volkszampler, you have to copy VolksZampler.dev into the Devices-folder.