For using DFG/USB2pro in Linux, we recommend Kernel 4.19. The driver is built in there. 

The input channel  Composite and S-VHS and the video norm NTSC and PAL-B can be configured with v4l2-ctl (paket v4l-utils), before starting your OpenCV application.

The possible commands are

v4l2-ctl --set-input 0 // that is Compsite
v4l2-ctl --set-input 1 // that is S-VHS

The possible video norms, depending on your analogue camera are:
v4l2-ctl -s PAL-B
v4l2-ctl -s NTSC

After configuring this, you can start an OpenCV cv::VideoCapture using program and you should see the live video.