That's one interpretation. If you're talking about the Nephilim, that's a difficult passage. There are other possibilities, though I admit it seems to indicate that fallen angels were mating with human females, and that's possible. It's probably the most likely interpretation of the text.

I can't imagine that fallen angels and human females would give birth to anything unless the fallen angels took on human form somehow. It actually says they took the "daughters of men" as wives, so that implies an actual relationship beyond mere sex, but in those days marriage was likely consummated with the sexual act without any type of formal ceremony or necessary paperwork. Though I'm no scholar on the culture of the time.

It seems to me that this would only be possible if the fallen angels took on human form and replicated the human reproductive system. I suppose that's possible, though it baffles my mind. We find in other passages of scripture that angels have taken on human form, so I don't know why that would be a problem. If it did happen that way, it would certainly explain why God was angry enough to destroy the earth, and most of the human race with it.

All of that is an aside, however. Within the context of Satan and Job, it's likely that Satan took on some form to interact with Job, as he did with Jesus. But this would be by God's permission. Satan would still be incorporeal, but with limited ability to appear in human form. In terms of moving "in and out" of God's presence, I'm not so certain he did so much as God has allowed him certain liberties with regard to access to the human race, even His own remnant.