If you mean the form to submit a new topic, then it would be possible, but is up to some customizations, specially in the database and submission/editing scripts. This is not available by default or any add-on.
In general, if you'd like to make it available to all visitors, then some way you should care about validity of keywords and their format. Only small percent of all forum posters know what it is about and how they should deal with it.
Also, I'm not sure - do you really think such keywords matter a lot? I suppose, Google uses its own algorithms of determining the nature of content, and it doesn't take closely the keywords tag at all.