What Iceberg said.
But... you'd need a lot of knowhow, a lot of time on your hands, and probably a bunch of Nokia N97s to brick in the process.
The biggest problem would be writing your own drivers for all of the Nokia N97 hardware - CPU, Screen, Sound, Memory access, storage access, input devices (keys, buttons, touch screen), cameras, sound input, etc etc. Nokia aren't going to do this unless they intend to use Android, so it would be up to the public.
Then, assuming the N97 has enough ROM storage, after a lot of testing and ironing out the bugs and coding fixes yourself, you might get it working!