I use the mod https://www.curseforge.com/minecraft/mc-mods/spawn-balance-utility to control spawns. Problem with the Creepergirl is, she appears in game just fine, but she does not in the mod's spawn control list, so I cannot spawn control her with spawn-balance-utility. Yes, I know the hmag mod has its own spawn control configs, but I need it in spawn-balance-utility for much more granular control.

The mod detects her just fine, because hmag:creeper_girl, MONSTER does appear in the AllMobEntities.rpt list generated by the mod.

Some examples how this mod controls spawns BiomeMobWeight.rpt:

301, savanna, biomeswevegone:araucaria_savanna, MONSTER, graveyard:skeleton_creeper, 20, 1, 4
659, badlands, minecraft:badlands, MONSTER, creeperoverhaul:cave_creeper, 25, 1, 2
1062, beach, minecraft:beach, MONSTER, creeperoverhaul:beach_creeper, 25, 1, 2

There's just nothing for hmag:creeper_girl anywhere in the list.

But other mobs from this mod are being added and can be spawn controlled, e.g.:

1067, beach, minecraft:beach, MONSTER, hmag:dyssomnia, 10, 1, 1
1234, forest, biomeswevegone:black_forest, MONSTER, hmag:ghost, 25, 4, 4
1235, forest, biomeswevegone:black_forest, MONSTER, hmag:kobold, 20, 1, 2
1236, forest, biomeswevegone:black_forest, MONSTER, hmag:lich, 10, 1, 1
1237, forest, biomeswevegone:black_forest, MONSTER, hmag:ogre, 10, 1, 1
1238, forest, biomeswevegone:black_forest, MONSTER, hmag:spider_nest, 10, 1, 2
1239, forest, biomeswevegone:black_forest, MONSTER, hmag:necrotic_reaper, 10, 1, 2

You can stop the creeper girl from spawning by setting creeperGirlReplaceChance entry in the config to 0.0 or by using datapack to overwrite hmag:creeper_girl_replaceables entity type tag to empty it, is that insufficient? (the same goes for zombie girl, etc.)
After that, I think you can set up a new creeper girl spawn using the spawn balance utility mod or datapack(forge biome modifier).

After that, I think you can set up a new creeper girl spawn using the spawn balance utility mod or datapack(forge biome modifier).

That's the problem. If it doesn't exist in SpawnBalanceUtility's BiomeMobWeight.rpt, it can't be added. I tried manually adding lines with like

1234, forest, biomeswevegone:black_forest, MONSTER, hmag:creeper_girl, 1000, 4, 4

It has no effect.

Apparently, the latest SBU does not work without renaming BiomeMobWeight.rpt to BiomeMobWeight.csv.
Please see the SBU description.