Lion is no problem but you must install the previous generation airport utility. On Lion this is simple.. and used to be readily available as a download.. however in 10.7.3 Apple blocked it. For typical Apple policy decisions they decided anyone who has a working express no longer needed or should use it.
How to load 5.6 into later Lion & ML
1. Download 5.6 for Lion.
http://support.apple.com/kb/DL1482
Click to open the dmg but do not attempt to install the pkg.. it won't work anyway.
Leave the package open on the desktop so you can see the file. AirportUtility56.pkg
2. Download and install unpkg.
http://www.timdoug.com/unpkg/
Run unpkg on the desktop.. If your Mac refuses to run the software, because it wasn’t downloaded from the Apple store, go to security in preferences and allow other software to work.. this is limitation of trade methinks. You can set back later if you like.
Now drag the AirPortUtility56.pkg file over to unpkg.. and it will create a new directory of the same name on the desktop.. in finder, open the new directory, drill down.. applications, utilities .. there lo and behold is Airport utility 5.6 .. drag it to your main utilities directory or just run it from current location.
There are even simpler methods.. just google.. install 5.6 airport utility. I would post you the URL but apple would then delete my post. It is and works absolutely without issues up to Mountain Lion. And you can run both 5.6 and 6.3 alongside each other. I do.. But for older model express and extreme units only 5.6 will work. But having 5.6 is helpful even for newer models.

You cannot extend wireless though with a G model express .. It is important to realise Apple made substantial changes.. so a G wireless uses WDS .. which is a totally different system to what the N and now AC models use.
If you want to extend by wireless buy a second hand Gen4 or Gen5 Airport Extreme if you want something cheap.. that is how mine is done.. one airport connected to second one by wireless..