I have looked up many homemade butter recipes. I know how to make plain butter. I have seen some recipes with herbs added and other seasoning. They don't say when to add the seasonings. Do you add them with the cream or after it is turning into butter?
I'd add herbs and spices after churning.
If you'd add them to the cream, you 'd discard at least some of it with the buttermilk and some more when "washing" the butter in cold water (depending on which method you use), which is a) wasteful and b) makes judging the correct amount very difficult.
When churning cream you'll eventually separate the fat from the whey, and after that you'll need to rinse the newly formed butter. If you add seasonings before that, they will be rinsed away and with it all the flavor, so I'd suggest to add after that.
Tradition way of preparing butter from curd or milk cream. I usually prefer to make butter from curd. Seasoning will works perfect as it is turning into butter and mix it.