You're right, apparently you can use Artnet with ShowXpress
but only when you have one of their interface connected and as per their website:
Each 512 channels dmx interface opens 512 dmx channels + 512 Artnet channels (or 1024 Artnet channels)
As far as I can tell there is nowhere you can set Subnet or Universe so you might want to contact Chauvet directly to find out how to do that. By default, Arena's ArtNet Input would be the IP address of your computer with Subnet = 1 and Universe = 0.