NOTE #1:
This code not only gives you all tracks from the Grand Prix mode, it even lets you access Story-mode tracks in practice / time trial mode!
However, it crashed upon starting a Grand Prix in Grand Prix Mode, most likely because the game is trying to load a non-existent level - so it only works in practice and time trial.
There are various removed levels, in most cases indicated by being called "null" or not having a map.

The code "SPEED-MODIFIER (GP-Mode)" was hacked by Y.S., ©ARcentral.net,
I just modified it a tiny bit to make it also work in practice mode, thus letting you access all tracks.

Now it works in practice- and timetrial-mode. Horray! ^^

This small modification might, however, on very rare occasions crash the game upon exiting the race.

The copyright for the code Access Beta All Cup goes to JAY007.
Link:
ARcentral.net