Some old posts before the blender SE like https://gamedev.stackexchange.com/questions/18312/how-can-i-see-the-height-of-an-object-within-blender?rq=1 don't really belong in the game dev SE. Is there a procedure for moving them, or marking them as duplicates?
In short you can't, they are stuck on game dev.
Any question older then 60 days can not be migrated. (Just a hard limit SE has coded into their system.)
Now for questions newer then 60 days. Most of the time site's migration paths will not be set up, so you can not simply vote to close because it should be on another site, and choose the correct site. (Mods can do that.)
Instead, you can flag for moderator attention with a custom message. Say why the question is off topic, and what site you are asking it be moved to (put the whole
http://blender.stackexchange.com so it gets automatically turned in to a link).
I've flagged on a few blender questions on SO exactly like that.
This is the message I used on one of my flags.
This question has nothing to do with programing, and should be moved to blender.stackexchange.com