Is it possible root my N1 with 2.3.3 without flashing custom ROM?
Yes. According to this XDA thread, you can unlock the bootloader with fastboot and then flash only a custom recovery. The custom recovery will allow you to put
su on your device.
You could also flash a stock 2.3.3 ROM that has been altered slightly (i.e., pre-rooted). For example, this one over at XDA.