Spiritual beings that, throughout Scripture, do very physical things, themselves, and have very physical capabilities....
Do the demonic possessions in scriptures result in anything supernatural in the humans that are possessed? What is there to preclude the same effect in physically altering the reproductive genetics of the one's they possess....being that angels themselves cannot procreate by God's design? There are demonic so-called "healings" of people's physical infirmities in some cases even today which are used to draw people down a false path and away from God. That would seem to me to be a form of genetic alteration or molecular cell manipulation.
Well that is not what the Bible actually says.
The Bible does say that "the sons of God came in unto the daughters of men, and they bare children to them, . . ."
There is no indication here in Genesis ch. 6 that any angelic being possessed and/or indwelled anyone.