Fred,
You are right, many generic plug-in power packs don't have any regulation in them, some proprietary ones do. There are many varieties, but the unregulated ones are just that. The DC voltage output is dependent on the load, and usually the AC to DC conversion is simply a single diode, or half-wave rectification. I use one for some lights on my layout, but I wouldn't use one for running engines. It most likely will work if you do, but I'd be afraid of the peak spikes that are higher than the "DC output" and could harm the motor over time. Some of these plug-ins are internally fused and if you get a derail and short the supply, that's all she wrote. You can take these apart to bypass the fusable links, but again, not a good idea.
The bottom line is, even if it works and you can adjust the rail voltage, it isn't worth the effort. You can get a nice MRC Tech 4 with acceleration/decceleration, forward/reverse, stop/start, output limit, AC and DC for accessories, all in a neat package for around $40, and the output is electronically regulated.
When I first got into this hobby, I was going to build my own power packs, after all, that's what I did for a living for years and I had most of the parts already. But I found a Tech4/350 on sale at my LHS and after testing it, I went back and bought a second one. I could build one cheaper with all those features as well, but I couldn't do it and have it look as nice plus I could spend that time working on my layout.
Mario,
That's a good assumption, but also be sure your engines aren't running hot either for the reasons I just stated.