Since you're in North America I'll translate from the british for you TSI:
SC: [9]N;
TN: Ns416555E
LG: <whatever your PRI line group is>
F: Dial
If all extensions have a DID number where the last 4 digits of the number exactly match the extension, you can simply use the above example (do the same for the other [9] shortcodes) in your system shortcodes list - if you have a handfull of phone that don't have DID's override the above with the same shortcode in their user shortcodes except giving the main number instead of 416555E after the s. Obviously you would change 416555 to your own NPA and NNX.
FYI DDI is the british term for DID, in case you weren't aware.
Peter