Somewhat. Women marching in armies is unlikely to have happened, but many sources do note women alongside barbarian tribes. Not usually in a front line combat role, but as support. Caesar, polybius, and Livy all note the presence of women traveling with migrating armies.

Some may be exaggerations, such as naked shrieking women throwing rocks over the heads of their men, but others are more realistic, of women and children following the army, as any honorable man would not flee battle through his family. This actually screwed the iceni, as they could not retreat through their families and were horridly slaughtered.

We have notes too, of women warriors, though I still highly doubt they would ever be in formation in a traveling army. That said, the taking of a town or city back then meant in no uncertain terms that every man would be killed, and every woman and child enslaved brutally. It seems perfectly reasonable that women would fight to defend their homes.

And keep in mind, Pyrrhus was famously killed when an old woman hit him in the head with a roof tile.

Anyway, to sum up: women in armies? Doubt it. Women in a Garrison? Very plausible. I'm at work on my phone right now, but am happy to dig up sources when I get home.