There are many articles on how to turn them into a fixed voltage lab power supply but making them variable is a different story. Unless you have the schematic, which are almost impossible to get hold of, and you really know what you're doing I wouldn't attempt it. You also run the risk of exceeding the component specifications and you'd certainly have to change all the electrolytics on the 12V rail because they probably be rated for 16~25V.
You could probably build a buck boost convertor and run it from the 12V rail but it'd be easier to buy or build your own power supply from scratch in my opinion.