Sorry, but there is no way to make an "extended" 460 cm IMCS 25 mast stiff enough
to rig correctly in a sail that needs a 490 cm IMCS 29 mast.
You can rig the 8.0 m2 Gator on your mast with the length of the mast and extension adding up to 495 cm luff length, but the top of the mast will bend long before it would using the correct IMCS 29 mast, so while it would rig up, the top of the sail would get loose and floppy and the lower panels in the sail would never get tensioned correctly.
Go ahead and try it (we know you will) but when you see how poorly it rigs, and if you try to sail it, how poorly it peforms, you will begin to look for the correct mast.
Hope this helps,