The second question is simpler to answer.
While editing video with 5.1 audio, you can add several tracks for any channel. After you add an audio track, and if you right click the source file in your project window you can choose to modify it, which allows you to assign it to behave as a mono, stereo, or surround track and whatever channel you want. For example you might have a mono wav file as a sound effect, you can tell it to be treated as a 5.1 file with only a center channel. The final exported file will mix down for example all center channels into one combined center channel, same for the other channels. While editing, pay attention to the master volume to make sure it doesn't clip, I think you can click View > Audio Meter to see the volume visually, if it goes into the red it's clipping.
For the first question,
Converting stereo to 5.1 can be as simple as putting a stereo track in the 5.1 timeline and exporting to 5.1, or as complicated as you want to make it for yourself. The problem is a stereo file only has L and R and will not have the information for channel separation you'd find in 5.1 and there's no way to create or recover that information from a stereo source file. So no matter what you do, your 5.1 converted from pure stereo will at best be "pseudo" surround, faking the desired effect by messing with volume to match the overall volume levels of an already existing 5.1 mix.
Getting the fake surround to be better or more authentic than whatever the program spits out automatically will take some research and decisions and probably some extra audio editing outside of Premiere. I can talk more about methods and ideas for this but it's pretty connected to the concepts discussed in the big thread about separating voice from music in this same subforum, so you might find more info there or possibly in other threads that also already exist, using the forum search function. This is pretty surface level explanation so if it's confusing in any way let me know where you want me to elaborate.
