I'd imagine the bererker type warriors were not large regiments of men. Less deserving of a unit than Gaestae, whom usually were the first wave of men. the Beserkers I'd imagine would be individual men whom just had a tendency to go into a blood lust in battle. Spread throughout the entire army, just regular soldiers, who might have done some amphetamines before engaging. There is no doubt in my mind they existed. But to limit them to just germanians? I do not believe that only Germanic people tended to go into a blood lust. Cuchulain, also known as the hound of ulster (for some reason made into its own unit, even though he was just a single man, and if he did infact exist he was WAAAAAAAAAAAAAAAAAAAAAAAAAAAAY before BI's timeframe) is a famous beserker, whom is credited to have saved Emain macha single handedly... but not before slaughtering some of the people who he was trying to save because he would descend into a blood lust and just start swinging at anything that was breathing. He was from Ireland.

Gaestae are also another form of what you'd coin as a berserker, they were warriors whom existed all over the loosely knit celtic empire. I would be surprised if there was not a single roman soldier who lost his head in battle, descended into a blood lust and started hacking at everything around him, especially if the formation broke and survival seemed bleak. "beserkers" are hardly deserving of their own unit type. Every man has a breaking point, you cannot at will go into your blood lust and begin hacking everyone to peices, its the individual inside your formation, whom is known for that or not, who it will happen to... unfortunately (really unfortunately, you have no idea how badass it would be to see this happen) we cannot have individual soldiers within a battle go into a blood lust and start hacking everyone to peices.