Strictly speaking, you *can't* convert anything else to Raw. Raw is just that; the raw data from your sensor, before all the modifications are made in software to convert the data to RGB numbers and adjust it to make it look good to the human eye. That's why Raw converters need to be different for each camera sensor; each sensor captures light a little differently (different number of pixels, different "full" and "empty" levels, and more things I don't fully understand myself), and the software needs to convert it to something that Photoshop (and the like) can use. Once something has been converted to anything but Raw, that conversion is already done and you *can't* back-figure what the data coming from the sensor must have been.
If your camera doesn't do Raw, the best you can do to adjust exposure after the fact is to set any in-camera image adjustments to the minimal-contrast setting, which maximizes dynamic range and minimizes clipping, and then increase the contrast to your taste after the fact in PS (or whatever image processing program you're using).
P.S. I'm going to be out of touch for the next couple of days, so any further questions this raises can hopefully either wait until I get back or be answered by someone else here.