Can one be a Hellenic pagan and worship, say, Thor?
There is nothing within Hellenismos that would prohibit a person from committing to the worship of more than one pantheon, if they felt called to do so. However, most Hellenes (and reconstructionists generally) believe that it is best to honor deities in culturally specific ways. In the case of someone called by Thor, it would make sense to seek out a heathen community that could support and instruct the individual, rather than trying to honor Thor in the context of Hellenic worship.