I hope you mean trawling… 
I agree, I wish people would use tags more. I don’t think you, as a regular member of the forum, can add tags to other people’s posts, though. It’s possible that this ability will be available to you as you “rise through the ranks,” as it were, to higher trust levels on the forum. Please note that this process is automatic, and is handled by the forum software.
But, what you can do is bookmark posts that interest you. When you’re viewing such a post, click the three dots on the mini menu below the post:
Next, click the bookmark icon that appears:
You can later access your bookmarks by going to https://forums.jag-lovers.com/my/activity/bookmarks
Or, to click your way there, click your avatar in the top right corner of any screen (1). Then click the profile icon (2). Finally, click the Activity menu item (3):
This will bring up the same bookmarks page as via the direct link mentioned above:
I hope this helps!
Gunnar