@ Gilles - To change existing entries, you would have to edit the campaign itself using any text program or word processor. I just use WordPad myself. To add new entries to either the random or scripted campaign, you would need to use the SHIII mission editor. You could probably get some advice on doing that over at the SHIII mission editor's forum.
As for ships showing up in the campaign, Mush Martin is correct in his description of generic entries spawning new ships. However, I would just like to elaborate and point out that this will be more prevalent for some ship classes than others in the stock game. For example, type 102 (freighter) and type 104 (coastal) vessels will spawn often for just about any nation as single contacts, and there are some generic entries for escorts, tankers (type 101) and passenger vessels (type 103) as well (mostly for the UK and US). However, I don't think there are any generic entries for capital ships, and some types of vessels have not been assigned to some rosters. For example, a new tanker will be seen in the campaign flying the US, UK or maybe the Greek flag, but not the flag of any other nation to which it has been assigned.
Iambecomelife's new Empire Class Freighter is a type 102, so it should be seen quite often as a single contact, especially under the UK, US, and Canadian flags.
