Custom fields to keywords

For my current project, I would need to copy the values of the custom fields of a gallery item to a comma-separated list of keywords of the same item. The best way could be, I guess, to modify the function called upon pressing the save button in the Edit Photo/Custom fields dialogue so that it, besides storing the custom fields, creates also the comma-separated list of the values of custom fields and stores them to the keywords. Since I am quite new to php/mysql, could someone help? Thanks in advance...