File conversion from mono to stereo or vice versa in various ways can be approached in Samplitude under:
File > Export > Mono / Stereo convert
Another approach is to right click the Phase Button (looks like the greek letter Phi) in the mixer.
This will open the Stereo Editor, here you will find options for mixing and copying the left and right side of a track.
If parts of a stereo track cannot be heard, it is also possible that one side is reversed in phase (also known as reversed in polarity, which is technically more precise).
To fix this, inverse the phase of one side, this can also be done in Stereo Editor.