No, nothing for the HR24. Unless they come up with a USB method, there's no jack for it either. You must be thinking of the H25 dongle. It connects to the H25 using a 1/8" TRS (headphone) connector.
I just tried mine (H24 and HR24), and it actually works pretty well anywhere on the same floor, but between floors not so much (but to be fair the house structure, including the floors above the basement, is reinforced concrete - it was designed to be the only thing left standing after a Joplin-strength tornado).
Edit: In looking for a pic of the H25 dongle, I found a replacement antenna for the HR20.. So it appears that does have an external similar to the VIP (with the coaxial connector).