Nokia Beta Labs has debuted a utility for both S60 3rd Edition and 5th Edition phones to enable the editing of words in your phone's predictive text dictionary, a long-awaited function. You are also able to import and export the custom dictionary, meaning you can transfer it from one phone to another. There's a video to explain what's covered and links below.

From Beta Labs:

 "We are pleased to publish Nokia Custom Dictionary on Beta Labs to share with the community and gather feedback. We designed this experimental application to address the wish that many S60 users have had: to be able to edit the list of words used by the phone for text prediction and transfer it between phones. 

Nokia Custom Dictionary does exactly that, and we’ve also given you the chance to import words from a text file.  This means you can quickly “teach” your phone commonly used words or even create specific word lists to be shared among a like-minded community, for example in the medical domain."

Download link - Feedback page