The TC is a separate deal but installing an Aspen or dual G5s (and the appropriate extras) will get you GPSS. Somebody here once did install the simple DAC GDC31 GPSS converter several years back. Given the newer options many shops may not be familiar with it, but it doesn't look involved other than where to install the box in the cramped space behind the panel. For all legacy autopilots what it does is provide an alternative input for using the transponder in HDG mode.
2002 DA40: MT, PowerFlow, 530W/430W, KAP140, ext. baggage, 1090 ES out, 2646 MTOW, 40gal., Surefly, Flightstream 210, Orion 600 LED, XeVision, Aspen E5