Starfleet Battles is an intensely detailed and competitive game. For the longest time, how ADB designed ships was a secret. A few years ago, the team took a shot at rules to modify ships. The rules were written, but then abandoned by the authors. The general problem was Starfleet Battles ships have a power curve, meaning the ships should have limitations and abilities based on speed. It is easy to modify a ship to reduce or eliminate a limitation.
The nice thing about ADB is they have a rich history of two-way player support. They post rules for players to try before launching a product. Very often those products are loaded with player created content.
ADB correctly assessed that modification was not good for the product line, but then posted it for player's home campaigns. Personally, I like to use it for freighters. Freighters have a long life and well, they are often used ships with odd histories. They also don't fight very well, so modifying one doesn't unbalance things too much.
The freighter above has an extra sensor box, hence the large antenna. As a backstory, the ship was modified by Starfleet and then they realized the hull itself caused sensor performance problems. That and the antenna took up cargo space defeating the purpose of the ship in two different ways. It also causes a strange shimmy when accelerating and a terribly annoying whistle in the comms.
The backstory behind this modification is grandma and granddad promised to give a grandson their ship to carry on the family freight business. The problem was, the family business wasn't very profitable, so granddad and grandma built a retirement module on the ship before signing over the title. Worse, grandma and granddad don't talk much any more, so everything is rather awkward.
The S3 rules for Starfleet Battles are very comprehensive and nice for the friendly game but really very terrible for a fair game. However, if you kitbash ships, they are a handy reference of what is technically possible via the rules.