Well, you could a produce a format and method that makes it easy to insert new units as they are developed - that wouldbecool
But to answer your query - community units is a mod that adds about 70 ships, mostly merchants - to your GWX install - personally i think it's a must have since it raises the number of merchant varieties to the point that recognition is genuinely challenging (i'll play with Weapon Officer ship identification the week after hell freezes over)
this mod is hosted by Jimbuna and can be obtained here:
http://files.filefront.com/Community.../fileinfo.html
As for the keenly anticipated Merchant mod, check out the sticky thread Merchant Fleet Mod Progress Thread in the mod section
http://www.subsim.com/radioroom/showthread.php?t=142285