Thanks for your comment. I don't use FSUIPC for this kind of settings, but I figured out something else, which works good at the moment.
In P3D, I have assigned the spoilers (arm) event in key assignments to switch 22/23 ( AT arm on/off) of the BTQ, which function I use in my MCP hardware.
The first time I started up the PMDG737NGXu, I got some warnings, but I clicked it away, without consequences.
The switch works and after several starts, it still works, so now I can arm the spoilers without deploying and after landing it will deploy automatically.
The speedbrake on the axis can still being used for reducing the speed.