How to add extension attributes to Product Model and Collection

At PriceRenderer we have MagentoFrameworkPricingRenderPriceBox::getSaleableItem

It returns MagentoFrameworkPricingSaleableInterface which is implemented in MagentoCatalogModelProduct.

Question is how to add Extension Attribute to this model to be visible at SaleableItem?

At repository it works perfect with MagentoCatalogApiDataProductInterface. But here if i go to MagentoFrameworkPricingRenderPriceBox::getSaleableItem, extension attributes are empty.