Bob is right..its called Reverse cross

Or ´Cross back `

But, as i learned by Don Hollingsworth – there is a natural barrier to the parents

Also it would be good you read, the post in the library..where some early breeders have done it and write about it – Because in the 2 generation of 2 parents.. there is a low fertility.. but we have found that growing them more years.. they get more mature .. so they might be able to make seeds..

Also you could learn to test the pollen ( also in this articles ) for fertility..as because you see lots of pollen it can be infertile.. and doing crosses with infertile pollen gives nothing.

Collect your own section of parents .. .make your own crosses.. raise theese – new generations – make selections by the best – choose your goal
Even it is a small section.. it will give you lessons about.. what to do

As practicing is the best way to learn