Custom IDs can only be added to an event when it is first set up.  These cannot be added once an event is put into Active mode.

To add Custom IDs: 
1. Go to Communities > Special Events > Friends Asking Friends > Kintera Thon
2. Hover over the event in question and click Website Info
3. Click Fee Structure
4. Click "Go" under the Custom ID section
5. This will open a new window where you can create the custom IDs, and the starting number