I believe this has happened. See the early chapters in the Book of Luke. It seems evident that immortals are capable of having physical children as well…consider Adam and Eve. I hope I will be blessed to give birth to children in the Millennium since I haven’t been blessed with that in mortality. It’s possible. I just don’t know if they will let me. I could adopt those whose parents aren’t worthy to raise them but those children would also be immortal so if I adopt immortal children or give birth the result is still the same.
I think you are overthinking the Millenium. Let’s break down your question.
Second, I don’t know which “they” would let you or not let you bear children in the Millenium.
What I do know, is that there will be a reign of righteousness on the earth as Jesus Christ rules as King of Kings.
I wouldn’t spend any time wondering about the mixing of mortal and immortal DNA in the Millenium.