Hey again, thanks for your reply!
After some research I gave it a try with my Hero3. I din't want it to loose it's origin function as waterproof actioncam, so I just cut off the front metal ring which holds the lens' plastic cover, put two M1,6 screws in the surrounding metal aera and grinded a common c-/cs-mount Adaptor down to 2,5mm, which can now be attached with screws.
As you said, the original lensholder has to be shortened a little to not cause vignetting plus I still need to attach a 1/4" thread to make it mountable and get still shots.
I ended up with a Pentax 6mm for 1/2" Sensors (thread has to be shortened about 1mm to reach infinity focus) and a B+W UV/IR cut filter. Although it's a high quality filter, it does not work well in this combination, probably because it's meant to be used on top of a common hotmirror. Although my field of view is only about 45 degrees, it causes bad blue/purple vignetting, especially on dark aeras. Bokeh and blurred parts doesn't look too good too yet, and I don't think its the Pentax itself.
I'm now about to try a Kolari Vision UV/IR Hotmirror, which was designed to bring full spectrum converted cameras back to visible wavelengths and take normal pictures, just like original hotmirrors do. Had to get it in the US, so shipping may take a while..
Without the attached c-mount ring still usable as original Hero3.
Exact focussing with the LCD Backpack is almost impossible. Luckily you can plug in a mobile hdmi Monitor for a bigger view.
Bad purplish vinetting in dark aeras. Focus on the crankshaft cover.
Hard to get a crisp focus with the tiny LCD.