Hi @rabol
For right now , you can create a attribute for pretax, so when product will be created you can fill amount in this field.
We have fired two event in function collectTotals() ( calculate total amount of cart ) named as
checkout.cart.collect.totals.before and checkout.cart.collect.totals.after.
You can listen this event and can add this attribute value ( pretax ) to cart amount.
Thanks