Although I have plans to make it easier, there aren't a lot of ways to mass import tags right now.
Also, when I add 'this file looks like this file–should they have the same tags?' to the program, your specific problem may be fixed in general.
Until then, I think you will have to use a script of some sort, and unless you want to get very complicated, you'll want to use python, so you can create a 'hydrus tag archive' using my code, which would let you import all the tags for the new files. I would do:
Create empty HTA
Export all files with tags in .txt files.
For each ( file, text_file ):
convert file to webm/mp4
figure out sha256 hash of new file
submit ( hash -> tags ) to HTA
Then manually import all those new files, and manually import the HTA to whatever tag service you want.
If you have experience with python or other general programming, I can walk you through it. It wouldn't be more than ten or twenty lines of code. But if you have no experience with programming at all, it might be more difficult.
In fact, now I think of it, I'll look into adding a checkbox to the import workflow that says 'import tags from a [filename].txt if one exists'. Since I added it for the export, I might as well allow the opposite. I might not have time for it this week, but it won't be too difficult.