In stock and Out of stock made to order products

Hello!

I was wondering if there is a way to allow purchases even if we are out of stock? Our business has in-stock items as well as a made to order program. We would like to be able to turn that product into made to order if we run out of inventory. Our workaround was to create a popup from requiring customers to confirm their understanding that the piece has a lead time. However, this doesn't work for product pages that have multiple variations. Any suggestions would be great!