To go from stereo to 5.1 properly, you need something called upmixing software.
Without that, I'd recommend leaving it as stereo and just setting your receiver to Dolby Pro Logic or DTS Neo:6 mode.
With 2-channel (Stereo) source material, I agree with Jim.
Now, I often create faux 5.1, when presented with 2-channel source files, but there is a fair amount of work, to get it to sound good, and then, it will fall behind mixing 5 - 6 discrete 1-channel files, recorded correctly.
The synthetic Surround features of a newer receiver will yield better results.
Also, to output DD 5.1 SS (DD AC3 6-channel), you would need the Minnetonka Audio SurCode DD 5.1 SS Encoder plug-in, or, and depending on some other factors, their SurCode DTS Encoder (not a plug-in).
I also produce and create music, digitally, and will do many of those compositions in 5.0, or 5.1 at the beginning. That is easy to deal with in PrPro, and output with either/both the SurCode DD 5.1 SS Encoder, or SurCode DTS Encoder.
I'd let the receiver handle things.
If I put my receiver to Dolby Pro Logic or DTS, will the sound come out of all 5.1 speakers?
Thanks in advance