If you build steel mills, will these mills produce steel without consuming ore, or will they consume ore(s)?
If they consume ore, then it seems we are moving an existing resource from one hand to the other, but with an additional step to build a steel mill to do it. If the mill automatically makes steel, consuming no ore, then it seems we are creating raw resources, the missing ore, that did not exist originally.
If the end goal of the OP is to create more resources, building a mine, instead of a steel mill, might be a cleaner design choice for the hundreds of thousands who buy the game and never visit the forums or the WIKI. Mines increasing resources seems intuitive, while steel mills which conventionally consume resources, seems counter intuitive in using them to increase a resource.
Either way, it may require a little more work than we first imagine. Since the games resources are balanced to something, choosing where and how much additional resources can be added to the game would require some wise decisions. If to many resources are added, or allowed in the wrong place, then much of the economic game will loss value, since without scarcity, resources have little value. Unfortunately, the game is close to this state already, in SP for major powers. It would not take a big mistake to make resources almost meaningless, game wise.