It automatically does that. There should be a toggle "Sell milk" on the cattle selection screen for that individual cattle.
Yeah, it's enabled but the only thing its producing is food units and the occasional bottle of piss. 
That's the only thing showing and I've tried both with Max Upgraded and UnUpgraded Farm. Produces the same thing.
And this was posted a couple of months back without an answer.
Thanks for all the hard work you've made in this mod.
Even after talking and telling the slave assigned as the farm manager to sell the milk? Haven't had any hiccups with selling milk for gold except for maybe some text messing up