Moving windows between GNU screen sessions
You can use injcode to move a process between ttys. To move process with PID run in target screen window:
$ injcode -m retty PID
However when you close the parent Screen session, moved processes will also terminate.
Edit: I was wrong, it works. Everything is described here. Suppose you want to move "htop" and only one instance is running:
$ injcode -m retty $(pgrep htop)
$ pkill -SIGSTOP htop
$ pkill -SIGCONT htop
then on the previous shell type:
$ disown