They do make them. I have seen them (complete mags) and could kick myself for not buying them at the time. hkparts used to have just the extended floor plates for the 9/40, but he doesn't even list them on his site anymore. I quit looking a few years back. Like I said, I'd love to have some if I ever came across them again.Those will NOT fit the OP's pistol; they are for the Compact magazines and the USPf .45. Basically, the USPf 9mm and .40 are the only current HK pistols which use a polymer magazine. All the other current production pistols take metal magazines.
I'm not saying that finger extensions for the polymer mags don't exist... just that I've never seen any.
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Just a FYI, I got to give two huge thumbs up to topgunsupply.com! I placed my order at 1159 10 June and I got my mag entensions in the mail today! I had never ordered from them before, but I'm sure I will again!