Combining tax inclusive and exclusive products?

I am just building an e-shop and for some reasons I would like to have products with both the VAT already calculated in and without VAT yet (in which case the e-shop will add the VAT itself and show the price including VAT). I have expected this to be trivial with just a single per-product checkbox telling the plugin if it should take it as it is, or also add the VAT by itself. But there is nothing like that anywhere. Also no combination of settings works for me. Is there some simple way how to make it happen?