Cross a Legbar rooster to a maran hen, and keep only daughters.
Cross those daughters back to a Legbar rooster (their father or a different rooster), and keep any chicks that are colored like Legbars. Those Legbar-colored chicks should be autosexing, so make sure to breed only from ones that ARE the right color for their gender.
If you breed those Legbar-colored chicks to each other, or cross them back to Legbars, they should all have the autosexing trait. If you want more Marans traits, repeat the steps in the first paragraph, but use one of the autosexing chicks instead of a Legbar this time.
(Where I said "Marans," many other breeds could also work.)
At each step, of course you have to pick the correctly colored chicks to make the autosexing work, but you want to choose the ones that ALSO have the other traits you want. So for example, raise extras of the correctly-colored pullets, and then only breed from the ones of them that lay the prettiest eggs. Or raise several of the correctly-colored cockerels, then breed from the one with the nicest temperament.