The short answer is no. The GRE manufactured PSR-500, and it's sibling, the Whistler manufactured WS1040, do not have the internal hardware needed to handle DMR of NXDN. So, adding that via a firmware update or upgrade is not possible. That also applies to the related versions of these scanners sold under a RadioShack nameplate.
The only current scanners capable of DMR are:
Whistler's WS1080, WS1088, WS1095, and WS1098. The older scanners from GRE, such as the PSR800, Pro-18, and the Pro-668 (made by Whistler, sold thru RadioShack) can add DMR if sent in to Whistler for an upgrade, which makes them essentially into a WS1080.
Uniden has a number of DMR capable scanners, if you purchase the paid upgrade. These include the BCD325P2, BCD996P2, BCD436HP, BCD536HP, SDS100, and SDS200.