In the object model, each SPList has an AlertTemplate property which you can get/set.

The AlertTemplate is CAML which defines what the Alert will look like. You would need to modify the CAML to include extra colums or data you want in the alert.

Here's an example I used to change the definition of an alert template on the Shared Documents library.

To create my .xml file, I previously set a breakpoint in the code and copied the AlertTemplate property's text view to a file.

using (SPSite site = new SPSite("http://myServer:7337"))
using (SPWeb web = site.OpenWeb(""))
{
SPList list = web.Lists["Shared Documents"];

XmlDocument doc = new XmlDocument();
doc.Load(@"C:\Temp\AlertTemplate.xml");

list.AlertTemplate.Xml = doc.InnerXml;
list.AlertTemplate.Update();
}