It would be nice an option to manually feed the cloud.
Another thought to your specific issue Puntocom, is that if you want to feed keyword phrases into page, you would do so by just using them in your posts / editing a specific pages post and enter the keywords into it.
If a thread goes over 10 pages, then every page will usually have a different contextual meaning at the mathematical level, or what is called keyword cocurrence and semantic connectivity, at the algorithmic design. These are what Google use in their algorithm. This mod already helps improve each pages unique ranking for terms already defined by the algorithm on an individual basis, not a thread basis.
If you wanted to change a pages meaning, inserting keyword terms into the cloud will not achieve that, as its bigger than one keyword phrase or a few at the footer of the page. If the page isn't getting found for the phrase / term, then it won't by injecting artificially either, even if you go back and insert it into posts within that page. You would have to change that pages complete semantic connectivity, which means the overall meaning of that page determined by Google using words, phrases, synonyms, etc.
This is why forums are so unique when it comes to targeting Google, because a specific thread topic can start at apple and by the time the thread has finished, its talking about the Audi R8. So page meanings change contextually across the thread, thus what each page will be found for as well.
This mod will help increase the rankings for terms the page is already being found for vs. what you are really asking needs the creation of specific threads / pages, that you talk about a very limited focus topic and ensure that topic remains exactly specific to the title. That is how static pages are done for targeting specific terms already, and have been as far back as I can remember.
I hope that helps with your specific issue on how to change forum pages dynamic.