1. I don't know... but I doubt it... what you are asking for is the equivalent of chroma-keying (most of the good approaches are patented, and therefore not {natively} in free, open-source software like OBS Studio) {.. but AaronD notes below, there are 3rd party plugins for this}
OBS Studio composites (joins) various sources. What you are asking is to create a video source.
My guess is that you'd use whichever software you choose for you virtual reality background (not OBS Studio), and then use output combined with whatever other sources (audio and video) you desire in OBS Studio. but I could easily be wrong... this isn't my area of expertise.