In order to add the headcrab over luma, you need to find your Common.szs file and replace tico.brres (luma's file) with the one here. If you don't already have a Common.szs file in your My Stuff folder, you can find it in files\Race.

Once again, if you replace barney with a character other than rosalina, the headcrab won't load as luma only loads whenever you play as rosalina.