What do you mean by "superimpose"?
Do you want a double-exposure? A Picture-in-Picture? Do you have a green screen shot that you'd like to videomerge with a key effect to give the illusion of a new background?
I would like a picture in picture but both are videos and not still images.
The easiest way to do it is to put your main video on Video 1 and your Picture in Picture video on Video 2 directly above it -- then apply the PiP preset (from the Preset category of Effects) that best represents the look you're trying to achieve.
I will try this and let you know. I've tried video merge but the effect was too busy. I will try the transparency next time and see what happens. I will let you know. I haven't tried the PIP yet. Too tired to try anything else tonight.