One of two ways, you could either open your save game, look at the name of a ship that's currently in the station and search for that ship in your save game's data, then convert that ship into your new one by changing things like the hull ID.
Example;
Here's the
KK Kamchatka II, a Novgorod-class cruiser.
Here's the
Kamchatka in my savegame.
A little bit of fiddling, and now the
Kamchatka is a Moscow-class battleship.
And here's the
Kamchatka in my savegame.
The other method involves diving into the core files, or mod files if you're tampering with a mod.
You'll wanna go to the data folder, then scripts, then world, then corvus. In the corvus folder, you'll find corvus.java. If you open corvus.java, it'll look something like
this. Now, if you're tampering with a mod that's already been made, you can just search for a ship that you know you always find in a station at the start of the game. Here, I'm using the Moscow battleship again. Duping that line there as many times as I want will let me have as many Moscows as I want in a station at the start of the game.
Another method is fiddling with a convoy to have a really high chance of delivering the ship you want. However, I don't know how to do that. I assume it'd be similar to fiddling with the generation files.
Hope that helped.