Welcome aboard, Matthias !
That 5 volt mic supply only appears on the "ring" contact of the beltpack's TRS (three contacts: tip, ring and sleeve) socket, so using an adapter to a standard "guitar-type" tip-sleeve plug is the way to connect what you have in mind.
Make this adapter a "cable type" rather than a rigid "tube" or "barrel", as the rigid types put far too much physical pressure and weight at the beltpack, resulting in all sorts of fractures and failures.
I do not know the actual figure for the maximum level, but I can tell you that these beltpacks are VERY hard to overdrive, as you will appreciate when I tell you that some guitars can deliver over two full volts of signal !
By the way, do not worry if your AKG C1000 microphone does not indicate much level at the "Audio" LED, just use whatever gain is necessary at your "desk" or PA mic input, because the clean and noise-free XD-V system will faithfully carry your mic's sound all the way to the XL connector on the back of its receiver.