Unless I am very much mistaking, guys, the OP was asking about ore, not terrain.
@ I_Am_Pariah: Whether or not you can retroactively generate ore depends on whether or not the mod that adds that ore offers that feature. Currently, as far as I know, only Thaumcraft (for aura nodes, special trees, obsidian structures, barrow hills, crystal ores, cinnabar and amber) and CoFH Core (for Thermal Expansion ores and vanilla ores) can do so. Xycraft could do it as well if Xycraft actually existed for 1.5.
As far as I am aware there is no way to retoractively generate ores for Forestry, Industrialcraft, Tinker's Construct, Factorization or others (but I don't mind being corrected).