Gameplay vs Lore
"All condor-class light carriers started their service lives as Tarsus class freighters. The conversion process is an involved and complex affair and requires proper space dock facilities.
Some in the sector argue even the Domain, with all their technological know-how, relied on conversions of this type to make more efficient use of a merchant fleet during times of escalated hostilities. Others point out that it is unclear how ubiquitous this approach was - there could have been fleets of Condors roaming Domain space, but there is scarce evidence one way or the other. Such details were all lost in the Collapse.
Nowadays the Condor-class sees heavy service in the Sector as the conversion is, for many, the easiest way to procure carrier capability for their organization. Even with limited flight deck space, the Condor gets the job done, relying on its tough armor, excellent bulkhead strength, and a respectable (for a light carrier) weapons package."
You in game get the Condor as a blueprint to build. But it's always just a refitted Tarsus.
So these already exist. What with ships like the hegemony auxiliary Hound being a supped up freighter. All pirate and luddic ships being militarised civilian ships. And the Militarized Subsystems mod letting the player do it too.
But why bother? Small military ships aren't hard to acquire if you need something to fight. And armed freighters like the Mule and Venture can be modified to be far better anyway.