Hi @AlexChris 
Beyond what can be found searching those keywords https://apps.shopify.com/search?q=backorder%20popup
This is possible with an advanced theme customization.
often repurposing a pre-existing newsletter popup or size-chart system etc for the popup/modal/overlay by creating a custom javascript event handler to trigger the popup when a variant added to cart event happens.
if a confirmation data needs to go through checkout see the tangential agree-to-terms customization for vintage themes well https://community.shopify.com/post/614939
If you need this customization then contact me for services
Contact info in forum signature.
ALWAYS please provide context, examples: store url, theme name, post url(s) , or any further detail in ALL correspondence.