You are certainly able to do excactly that on your MR-8 - this feature was my main reason for buying one myself...
Given you have transferred all 8 tracks from the MR-8 to the PC and mixed them all down to a (mono) wave file, do the following:
1) Rename the mixed down track to "tr08.wav"
2) Create a fresh project on the MR-8
3) Transfer the wave file to the MR-8 via USB (I assume you already know how to do that, otherwise consult your manual)
4) On the MR-8, the mix will now sit on track 8, ready for more tracks to be overdubbed (provided you have enough space left on the CF-card)
Note: Putting the premix on track 8 allows for greater flexibility by freeing tracks 1-7. You will particularly want tracks 1-4 to be free. Alternately, you may make a stereo mix, split it into two mono wave files named "tr07.wav" (left) and "tr08.wav" (right) respectively.
Please feel free to ask if this isn't clear to you.
Happy trackin'!
regards, Nils