I don't like installing Chrome extensions from 3rd parties because these extensions can literally see everything you do in your browser, can grab usernames and passwords, etc. without your knowledge. (I once wrote Chrome extensions. It's scary what you can do.)
There are a ton of "merge windows" extensions that let you combine tabs from multiple windows into one window. If you're not paranoid, any of these will probably get the job done.
If you are paranoid, there is a "merge windows" example extension in the developer docs. The code is really simple (easy to read through to see if anything nefarious is going on) and it is installed as an unpacked extension so there are no automatic updates that could introduce unknown code.
Here's a backup of the code in case Google takes this out of the docs someday.
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