Yes you can. (I'm assuming that you already know that it's not just the opaqueness, you will also need to work with masks and sometimes layer modes).
It sounds like you are just learning about what HDR is and what tone mapping is too.
HDR is the technique of photography that involves combining multiple exposures in a 32 bit color space. The 32 bit color space is well beyond what your eyes can see, billions of times more information than your computer screen can show and is totally impossible to print.
Tone mapping is the technique of bringing that information into a usable 16 bit color space.
It's considerably easier to work in a 16 bit color space from multiple exposures or multiple processings of RAW.
The biggest problem that you will experience in PS 7.0 with different exposures is that each exposure will likely be just a few pixels shifted away. There are techniques to correct for this, but it can be a real pain, especially if there are distortions in your lens. For this reason, most people just work with two or three different renderings of the same RAW file instead of actual multiple exposures. You won't get as much room with this technique, but it's often enough to replicate the dynamic range of our eyes.
CS3+ has a specific function to automatically nudge multiple exposures so they match up.
There is a technique that you can use to line up layered exposures, I believe you can use the layer mode called 'difference'. Sorry I have forgotten a bit. The Radiant Vista has a video tutorial on the method. It's a bit older and I think it's made for CS2, but the technique works in 7.0 too.
