Good question, Gargoyle. I thought I had more information on it, but I found only one book that discussed the topic - The Texas-Mexican Conjunto by Manuel Peña. Peña says tht by 1850, Germans have begun to establish themselves in Northern Mexico, and to assume Mexican citizenship. There was an especially large German community around Monterrey, and many Germans were employed in that city's brewing industry. Peña says German workers introduced the accordion to Mexico, and it's my understanding that they also brought the polka.
-Joe Offer-