Quite simply, I'm afraid you can't.
When breeding with Pokemon of different species but of the same egg group, the offspring will be the first stage of the evolution line of the female. Unfortunately due to breeding mechanics, Hidden Abilities can only be transferred from the female in cross-species breeding:
If the male is bred with another Pokémon from the same egg group,
Hidden Abilities will not be passed down.
This means you would need either a female Feebas with its HA to breed with male Marill, or a male Feebas with its HA to breed with a female Feebas. Alternatively, either gender Feebas with HA to breed with Ditto.
Basically, I'm afraid you need a HA Feebas to start with :(