Wait until you actually have an offer in your hand. Once you do, review the offer. If it's not an offer you would be willing to accept, you can reject it or attempt to negotiate with that company. If it is an offer you would be willing to accept, then you can go to other companies that you are interviewing with, inform them of the new timelines, and see what happens. Some companies may be able to accelerate their process to meet your new timeline, others may not. You can see if the first company that provided you with an offer can extend their timeline. But any negotiations may result in an offer being retracted.