Merge Separate Windows (and tabs within) into one in Sublime Text 3

[Not sure for other OS but for MAC it is easy and very useful]

In mac, open sublime,

click Window -> Click Merge all Windows

Also make sure that this won't work if you just have one window open which is pretty obvious.

This isn't exactly what you're asking for but I think this is close enough:

Drag the tab from the "source" window to the "target" Window and hover near the other tabs in the source window. Then drop it there. Voila. You've moved one tab. Rinse and repeat for the other tabs.

I have created a sublime package to help solve this problem called Merge Windows.

It can be downloaded from github or through Sublime using Package Control