Is it possible to do this hack without flashing? Like installing some package? I dont want to flash it, because I'll lose some apps/settings and time, and If I want 600MHz stock kernel again, I'll have to flash it again. (And If 900Mhz does not work for me, or If I want to switch to 800MHz, I'll.. flash it again.)

In desktop GNU/Linuxes, It is possible and easy to use different versions of kernel. (I had xx.22 to xx.34 Linux kernels on my Ubuntu)