I tried with the other mixer-models too, but with 4896 it worked the best.
1. Make a copy of your STM 4896.dev in your devices-folder and rename it to STM 4896-XTC.mdl
2. Load this new ...mdl into your XTCProject.pro.
3. The Mixer-Window has to be open when saving the project.
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 disappeared.
That's it
5. You can use everything in the mixer 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 in the mixer.
Workaround: First load optimaster as XTC instrument in Cubase, then you can load it in the STM-mixer too.