O_o ... err... HOW did Ranma get to be able to produce milk? Did someone
get her pregnant, too, and the kid died or something? Or is that Ranma's
kid, and Akane got pregnant, but the kid died? Ummmm....???

Since this is Ranma 1/2, we can use:
1)  Its a sympathetic hormonal reaction - akin to why women who live together 
start to have their periods coincide.
2)  Its a feature of the Jusenkyo pool that Ranma fell into (when all else 
fails, fall back on this one).
3)  It is Ranma's child, so the Akane giving birth triggered the ability in 
Ranma's female form (and if you thought Akane was going to pass that 
opportunity up, think again).
4)  This is Ranma 1/2 - We don't have to explain anything.

In other words, you didn't think about that?

Kun-chan...