I'm connecting a trans cooler and would like to extend the hard line to the cooler with additional hard line. I got some 5/16 line, but need either a connector or ferule or something. The male of the extension fits the female of the existing line. I tried a 5/16 ferule, but that didn't work. In the past I've used a short rubber line with 2 clamps to connect them but this time I'd like to keep it all hard line. Anyone know how to do this?

Howdy neighbor. Try to get a male to female adapter, might have them at the auto parts store. Do you know where Jungle George's is? Hawthorne auto parts. The place is a hole in the wall, but he has or can get what you need.

Ace Hardware has a good selection of brass adapters.

I used Ace Hardware for all my adapters and fittings also

I found the correct couplers at Autozone. You will need to confirm the thread on the fittings on the hard line.