Here's a link to Conflippers thread, IIRC when I used it, it lagged at first due to the number of cities loaded, but it did speed up. There is also a program for adding in even the smallest town, but I don't have it with me at work or on my mobile, so if you need it, I'll post it up later. You also have to use total commander to change a manila file from read only. If conflippers app doesn't work, let me know and I'll get you the info on adding your individual town.
http://forum.ppcgeeks.com/showthread.php?t=81108
Edit: I just took a look at the cab that Oditus posted above, I believe that is the editor I used, and IIRC you need to go into /windows w/ total commander and change the attirbutes for 2330fc3c_manila to archive and system, NOT read only and dump it back into /windows. Then you can use the editor without problems.