You certainly could add cantilevers to a steel bike, here was my conversion;
http://www.bikeforums.net/showthread...=#post12712430
I am a bit uncertain that Shogun would make a decent conversion. Any idea if it is Cr-Mo or is it Hi-tensile steel? Hi-10 will be heavy and probably not strong enough, I have seen some chepo CX conversions where a low quality frame simply disintigrated under the stress. I dont know this Shogun model but it looks to me as somewhat lower end (stamped seatpost binder, minimal finish work on the chainstay/seatstay junctions at the dropout). Rack eyelets suggest possible touring geomotry which may be too sluggish for actual CX racing.
I would suggest you check the mud clearance, weigh the frame to discern if it is decent tubing or a POS. The TT measurement sounds way off, measure the ST and TT center-to-center and try to get an idea of the frame angles. (a side picture with the TT horizontal would help). If you do go forward with it, I would probably suggest you first build the bike up as a pseudo-CX (tires at least) and ride it a bit first with the existing brakes to see how it fits and performes before you spend $$ on a canti-boss conversion. Depending on what it cost you to have the post added and what you spend on paint, it might be just as cost effective to just buy a new nashbar or BD chinese frame and have a better bike in the end. Dumping money into a conversion of a mediocre frame will just result in an expensive mediocre frame.