The evidence is in the genes...and these Jews would likely NOT have had rabbis with them to convert the women.
This assumes a lot, in particular the “profession” (like every other profession) of rabbi was a lot less formalized back then than now.
A Beit Din, technically needs, what, three adult jewish males? Sure, there is almost always (now) one man who is, by profession a rabbi and it is very formalized, but in the pre-middle ages? Who knows?
Body of water for the mikva/tevilah, a kabbalat ol mitzvot (agreement to do the commandments), taking the name (probably Ruth or Sarah). . . . no mila for the females -— so I don’t see how this would be a problem.