Presumably with help from Myrmidians.
Let me explain the method to my madness.
My thought processes went like this:
-This sounds awesome, so how can we explain it not having been a thing so far, especially since bull warriors are right
there?
-Answer: They have not been developed yet, or it is a thing from outside the empire.
-Hmm, bullfighting has a fucklong history in Spain and Estalia is strongly spainflavoured. not to 1=1 levels, but strong flavour nonetheless.
Yeah, I can see there being a not-Pamplona in Estalia.
-Since it is not-Pamplona of course they would have a yearly running with the bulls.
-Since this is WHF, of course there would be some madlad going - yeah I can ride that. I mean, we live in a less bonkers setting, and there are people like that here. probably in your very town. But, since it is WHF, some of the madlads can actually succeed. And continue succeeding to the point that they can go on and teach others to do so.
-And wouldn't you look at that, Freddy just recently made this grand deal with the imperial branch of the Myrmidian church, which would see a lot more of Myrmidian presence in Ostland.
-All it would take would be a non-Pamplonan expat, maybe even one who knows a thing or two about breeding and training war bulls, to reminisce before an appropriate public to get the local madlads getting ideas.
Reformatted to be less of a wall of text.