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It depends upon the module. For instance, SmartSection, one of several content managers, has a keywords field for each and every article. It fills the $xoops_meta_keywords variable with the value placed in this (and if left blank, it automatically fills in some keywords for you). Hopefully we'll see this in more and more modules as keywords are (somewhat) important for search engines. However, I will note that some search engines are beginning to eschew (avoid) use of the keywords field for other things like number of links to that page with appropriate keywords, date of material, etc., but generally still a good idea.
So, nothing automatic, per se, but capability exists.