The ACS doesn't have any accomodations for ARS or least cost routing.
Maybe it's time to move up a Legend 7/Magix. With ETR cards, the same desk sets can be used. Your ACS equipment maintains a high value. This along with annualized toll savings could offset the transition while providing the larger system features you're likely to desire.
While you could look at frontending the system with 3rd party dialers and devices, I think you/your customer will find them akward at the least, to a hassle.
If you simply are looking to add/strip dialing digits on a 1 for 1 line relationship, this might work ok, but if you're looking to route calls on various lines or trunks, these switcharoos will result in line usage lights not matching up to call progress tones, busy lines, etc.
Best of luck