Yes you can do this; it is not a limitation within UBB.Threads, it is a limitation within MySQL itself and FullText searches and your webhosts configuration.
This relies on the minimum search length set by your webhost; the default I believe is 3 characters, which is why this error comes up.
i believe rick was planning to update the search system to have an option to use the more powerful "full text" searches or the "old" method of searching based on the admin's choice; but i'm not sure when this was schedualed to be updated.
