In 6. 10 at least, I was able to retroactively increase the fuel storage of my already built Freighter class ship by hitting New Armour in the Class Design window - which reactivates the Unlock option, allowing me to add more components, which then got added in-flight at no additional cost. Increased storage size from 100,000 to 300,000, if I remember right. (The extra storage space came empty, of course, so the damn thing remained stuck in space. )