The
Knox is a fan creation (one of Todd Guenther's) that has a slightly smaller hull than the
Miranda.
http://www.shipschematics.net/startrek/images/federation/frigate_knox_proposed.jpg
Turning a warship into a freighter might be difficult. Recently, surplus USN
Oliver Hazard Perry frigates, probably the most modular and generic surplus warship available (and perhaps a good analogy for
Miranda in other ways as well), were considered for conversion into Coast Guard high endurance cutters, a minor change in role that would mainly entail abandoning some original operating modes and systems, not adopting new ones. That proved to be too complicated and expensive... Not due to the costs of operating a former warship as a USCG vessel, but due to the costs of conversion alone!
To turn a
Perry into a freighter would mean major reworking of interiors, resulting in a ship that was struggling with the weight of the load and consuming excessive amounts of fuel in doing so. I guess a
Miranda would make for a piss-poor freighter, too. Some sort of a special military transport, though... Perhaps a vessel that can carry a smallish load very fast and under fire, for assault or siege-breaking purposes? The navies of WWII and the Korean war built those out of surplus destroyers.
We might look farther back, too, and decide that the torpedo-less
Miranda we saw in "Unnatural Selection" was a transport in the same sense as the sailing ships of the 15th century were. That is, armed to the teeth (if only with phasers in this case) because hauling of cargo in the frontier wilderness is more dangerous than fighting in a battle line...
Timo Saloniemi
The 
