In GarageBand 2.0, the end-of-song marker cannot be moved beyond measure number 999. However, it is possible to record a Real Instrument track beyond that point, but if you do, the portion of the recording that extends beyond measure 999 will not be playable. If you want to use that portion in your song, split the region at the end-of-song marker (at the end of measure 999) and drag the overmatter into any area between measures 1 and 999.
Tip: Consider creating a new Real Instrument track to move the split region into - otherwise, you might overlap regions.
In GarageBand 3.0 or later, this limit increases from 999 to 1999.