I believe you need to use the Chemidoc version not the standard Geldoc. I have done it with the Chemidoc and it works well. You can auto-expose or set the time interval manually similar to putting on film for various exposure times.
You need to talk to Bio-Rad to see if you can upgrade the geldoc to the chemidoc if you want to do chemi-illuminescent blotting (ECL). It is more than a software upgrade between the two systems. There is a lot of info on Bio-Rad website.
Unfortunately, you cannot upgrade the GelDoc to the ChemiDoc for your chemiluminescence western blotting experiments. Please have a look at our latest imager, the ChemiDoc Touch.