Can we find a string all post and insert a tag to all of them?
If you’re talking about matching a string in a post, then tagging the topic – yes, that’s possible, but it would need to be via a script. Not possible via a current rake task.
You can do it from the console once, as @justindirose suggests, or if you want it to happen all the time you could do that with a plugin.
Can i offer this plugin?
You can see Beginner's Guide to Creating Discourse Plugins - Part 1 or post in #marketplace if you have a budget.