Congratulations on receiving the invitation for the first-round interview with McKinsey!
To address your concern, McKinsey, like most consulting firms, tends to be understanding and flexible about interview scheduling. It's completely fine to ask for a reschedule, especially if it's only for an additional week. You can mention a scheduling conflict or that you require a bit more time to prepare, both of which are legitimate reasons.
That said, make sure to be polite and professional when making this request. Thank them for their understanding, and express your eagerness for the opportunity to interview with them.
Remember, however, that the impression you make during your interview will far outweigh the impression made by requesting a schedule adjustment. Therefore, ensure you utilize the extra time to prepare thoroughly so you can put your best foot forward on the day of the interview.
Best of luck with your preparation and interview!