It is possible to do this using what is known as a WDS setup, but the huge drop in performance on your network will likely far outweigh the gain of a bit more wireless coverage.
Consider what will happen if you try the WDS setup:
1) Your AirPort Express "n" will automatically drop down to "g" wireless levels
2) The bandwidth of the entire network will drop by 50%, so in effect you will have a "g" wireless network running at half speed. As a reference, your "n" Express will normally run at about 5 times the speed or more of the WDS setup.
3) The configuration to do this is frankly difficult and frustrating for most users. It's easy to make a mistake and impossible to recover without starting all over again.
I would not recommend this, but if you want to try it anyway, Apple's instructions for this are here:
http://support.apple.com/kb/HT4262