Many times a brass insert is cut with the inverted flare seat and pressed into a bored recess in the part. I know that master cylinders are made that way. That type of part can be made on a lathe.
How many of these parts are you making?
If you are doing a bunch, like Dugg said, a custom cutter may be the way to go.
You'd try one of these guys who do custom tooling.
( I never used these particular people, I just googled "custom tooling"
is a company I found that made a tool called a "cone cutter", this one is a 10* but it looks like it's the right idea.
IIRC, rutland and enco offer custom services as well.