FZF to select tmux windows and move them to another session. You may select multiple windows.

tmux.sh

# this in your tmux config

bind M run-shell "source ~/.tmux/functions.sh && move_windows"

# this inside ~/.tmux/functions.sh

#!/bin/bash
#
function move_windows() {
    windows=$(
        tmux list-windows -F "#{window_active} #{window_index} #{window_name}" | sort -r | awk '{print $2 " : " $3}' | fzf-tmux -p | awk '{print $1}' | sort -r
    )
    if [ -n "$windows" ]; then
        session=$(tmux list-sessions -F '#{session_attached} #{session_name}' | sort -r | awk '{print $2}' | fzf-tmux -p | awk '{print $1}')
        current_session=$(tmux display-message -p '#S')
        echo $windows | xargs -n1 -I {i} tmux move-window -b -s "$current_session":{i} -t "$session":1
    fi
}