When you're talking about something that draws megawatts existing transformers are pretty irrelevant because you're going to run high voltage lines directly to the site itself and install new dedicated transformers on site.

What's more common is that they don't have the transmission capacity itself, but that one's pretty easy in this case too, because what that means is that you have an existing transmission line which is already near capacity with generation on one end and customers on the other. So then you just build the data center on the end of the transmission line where the generation is rather than the end where the existing customers are, at which point you can add new generation anywhere you want -- and if you put it near the existing customers you've just freed up transmission capacity because you now have new customers closer to the existing generation and new generation closer to the existing customers.