You're really asking two questions here, and I'll address them separately.
Can a "modern" civilization have a polytheistic religion?
Assuming that you mean "scientific-based" when you say modern, then there's no reason to believe that there's an inherent conflict between polytheism and a modern-level civilization. There are certain aspects of a polytheistic religion which don't translate well, but overall the concept would hold.
Specifically, the most likely scenario is that deities as explanations of natural phenomena will get marginalized, but patron deities will stay around and get more established.
For example: The ancient Greeks attributed lightning to Zeus. A modern civilization would know that lightning is a natural phenomenon caused by charge differentials between the sky and the ground. So while phrases like "Zeus' wrath" might be used to describe the impact of a lightning bolt hit, no one will actually believe that Zeus is flinging bolts around. However, Zeus was also the god of hospitality and oaths (among other things). His name would still be meaningfully invoked before swearing an oath, and religious hotel managers would almost certainly pray for his favor.
Likewise Apollo-as-sun-god wouldn't get more than nominal lip service, but Apollo-as-patron-of-music would be invoked all the time.
It's worth sidetracking to point out that just as today many Christians only pay nominal lip service to God and/or Jesus, but often use them as exclamations, a "modern" polytheistic religion would likewise have many people who are technically adherents but don't really believe, but still invoke the appropriate gods for cursing, swearing, or just exclaiming in surprise.
(I do not address Hinduism here, as it's highly complicated and can't just be called "polytheistic". I also don't specifically address religions, such as Shinto, where there are no "gods" per se, just spirits of greater or lesser power - although the principles I laid out above would probably apply there as well.)
Can a polytheistic religion incorporate a monotheistic deity?
Assuming that you mean "monotheistic" in the sense of the Biblical omniscient, omnipotent, omnipresent, invisible God, then there are a few ways this could happen, depending on the relative strength of the monotheist believers' culture vs that of the polytheist one, but it's not a very likely process. A henotheistic deity is much more likely to get absorbed in this way, but that's not what you asked about.
If the monotheistic culture is stronger, the most likely scenario is that the polytheistic deities would be absorbed into non-deific roles in the monotheistic religion. Beelzebub is a great example of this - originally the Canaanite god Ba'al, he got co-opted into a prince of Hell. Santa Claus is another, incorporating elements of the Norse god Odin. Thus the pantheistic gods could become angels (or the equivalent) in the monotheistic religion, serving the higher god. However, it'd only be a matter of degree and semantics for them to still be deities in their own right (even if subordinate ones). In this case, the monotheistic god becomes the chief god (I'd guess in a "hands-off"ish way, with the former chief being the much more visible second), but the religion is still polytheistic.
If the monotheistic culture is weaker, the outcomes I can see would be for the polytheistic one to conflate the other deity with their king of the gods (Zeus -> Jupiter), to assign the monotheistic one a specific role ("You're a scholarly people - your god is the god of scholars"), or to simply say "It's a valid deity, but not part of our pantheon". That last could potentially be incorporated as being a god of a place ("Make sure to offer a prayer to XXXXXX as you travel through his lands.").