They can't get enough of the divas on ''Will & Grace'' -- and we don't mean Jack and Karen. NBC announced that Jennifer Lopez will return as a guest star in the seventh season premiere on Sept. 16. The episode will continue the storyline from last season's finale, in which Karen (Megan Mullally) asked Lopez (playing herself) to perform at her abortive Las Vegas wedding. Now, J. Lo is back home in New York, resting after a tour on which Jack (Sean Hayes) served as a backup dancer. Karen annoys J. Lo again, this time by using Jack to push a song Karen wrote, called ''I Hurt, Too.'' As NBC announced last week, the Lopez guest spot will be followed on Sept. 23 by Janet Jackson's first sitcom gig in more than 20 years, with Jack shuffling his dancing feet over to Jackson's tour. No doubt Britney Spears' agent is next on the producers' phone list.