After being searching over the internet for many days, I thought this could be a good option:
Dialog | jQuery UI
Draggable | jQuery UI
These two links give good examples so I was trying to modify the code myself, I managed to find the template "thread_create" "PAGE_CONTAINER" and add/change the css and script accordingly, but the attachment editor won't showing up afterwards.
I think it would be nice to have this feature implemented in the up coming version of Xenforo as it's really handy.
Any help are much appreciated.