Adding mods with world gen after the world has already been established is an old problem. The good news is, because its an old problem, most mods have a solution for it. That solution is a retrogen option in the config. Enabling this feature will force the world to spawn the appropriate ores (and sometimes world structures) wherever it deemed the correct conditions.
In a pack such as Regrowth, this becomes a bit trickier, as world generation of ores is intentionally disabled/suppressed. If you were to add a new mod (say Big Reactors, as that was a previous example), your best bet would be to edit the json files created by COFHcore which govern ore spawning to blacklist the Overworld for Yellorium while whitelisting it for the Nether.
If you wanted it to fit in smoothly, you would have to take the extra time and effort to edit the Minetweaker scripts to balance it into the pack. This includes, but is not limited to, configuring Magic Crops to allow Uranium (Yellorium is oreDict as Uranium) as an allowed crop, and then Minetweaking the recipe to include using the Runic Altar and appropriate Botania runes (due to the nature of the mod and the tenor of the pack, I would gate it pretty deep, requiring multiple Extreme Essence). It might even be a nice touch to tweak the recipes to require alloys from Mekanism or other late game items (Turbine Controller needing Voidmetal or somesuch).
Then again, why bother with all that when Regrowth is already an amazing and balanced pack? Realistically, the only mod I would even think about adding in is Logistics Pipes, which relies only upon BuildCraft (already included in the pack) to be functional. If you were to tweak the basic pipe recipe slightly, it would have a compounded effect, and most pipes already require an Assembly Table if I'm not mistaken (if I am, its another tweak I would make).