Never used prefixes until now, Need help tagging

[RamdonGhai]

If I had a for sale forum on my site which listed cars by manufacturer, in separate forums... Is there a way I can automatically tag all threads in the "hondas for sale" forum with the prefix [honda] and combine all of the forums into one forum called "cars for sale" And then create the prefix option so all new threads will have a prefix from this point forward

Thanks