You can't just pull keyframes out of thin air, they have to be defined inside 3dsmax on the timeline, and it is those who are exported. They don't have to be set at a fixed sample rate, you are free to create a thousand key one second, and none the next. It is completely up to you.
Have you had a look at the exporter, IGame, that ships with the maxsdk? All the data is exported to xml files, so they are easy "debug", ie you can see the raw data, and don't have to write a importer "in the blind". Helped me a lot.
I understood the keyframe you descriped and what I want is export the keyframes defined in 3dsmax.
Actually my 3dsmax5's maxsdk didn't have IGame liberary files and samples. But I found the its .lib file, .h file and .dll file from one open source project and I used it in my project. I knew how to use GetFullSampledKeys function but it only can export the fixed sample rate frames. I just couldn't get the key frames defined in the 3dsmax's timeline. If you can send me the exporter you said, it would be great. Then maybe I could figure it out myself.