This is interesting, because it was actually kind of the other way around. The Romans typically added other peoples' gods to their pantheon. This was rationalised through 'aspects'. In Greco-Roman Paganism the gods could have different aspects. For example, in a prayer to Diana, Catullus calls on many of her different aspects for help:

You are called Juno Lucina by those sufferingthe pains of childbirth. You are called powerful Trivia and Luna with the counterfeit light. You, goddess, measuring out the path of the year at a monthly pace, fill up the home of the farmer with good produce. May you be hallowed by whatever name pleases you, and, as you were accustomed to do in the past, may you protect the people of Romulus with good aid!

Thus you can see that one God could appear in many different forms. When Romans encountered other beliefs, they could be incorporated as forms of their own Gods. When encountering Egypt, the 'primary' deity, Ra, might be considered an aspect of Zeus/Jupiter. Anubis might be considered an aspect of Hades. The other important thing is that there was no one to oversee this or establish an orthodoxy. Greco-Roman Paganism had no orthodoxies or doctrine, so one Roman might see Anubis as an aspect of Hades, one might see it as something different, and they are both still correct and part of the same religion.

Where Romans would have forced their religion onto other communities is through ritual and sacrifice. Every community and all the people in them had to perform certain religious rituals, which were very important to the Roman state. This was part of the problem with Christians, who would not perform the required rituals.