Considering your situation you have only a few options:
From a stereo mix you may extract 4 audio tracks that compose the stereo mix:
Left and Right can be extracted using PAN
From the classic M/S Matrix:
Left = Mid + Side
Right = Mid − Side
So, Left - Right = Side
Left + Right = Mid
Using Ableton you can use EQ8 and set it up to M/S
Still over at the utilies section there's a diference and invert plug in to switch the channels.
In other DAWs:
1.) Extract the audio into 2 Mono Channels.
2.) Invert the audio from one of the channels.
3.) The Result is the SIDE audio.
4.) Using the SIDE audio, invert it and add it to the right + left side - or even to the single left or right.
5.) The result is the MID audio.
But to be 100% accurate and honest.
On the market, on the moment, there's only one solution that will be able to do what you want with hi quality.
If you never heard of it you should check youtube for a melodyne dna demo or read this.
It's possible to extract ANY piece of audio from a Mixdown, you can even change keys completly or generate a new instrument from a sample.
Check this video:
It shows how you may extract some information from Lucy in the Sky with Diamonds.