Well, if you have only the models, you would need to mod in new hull entries to the game's data that use them, or manually make the ship designs (the latter is metaverse-safe if you do it right--that's how the meta-safe version of my mod works).
The shipcfg method is a simple and clean way, though the initial setup can be repetitive since you have to make two files for each ship.
First off go to DarkAvatar\Data\English\ShipStyles. Open one of the styles, and for now just change the name on lines 3 and 30, and set the Prefix setting to something unique. Then save this file with a name to match the name you put in on lines 3/30 in My Documents\My Games\GC2DarkAvatar\ShipStyles. Dump the models and any associated textures in DarkAvatar\Gfx\Models.
Now that the style is there and the models are in place, we get to the fun part.
For each model you want to add to the game, create a file named [Prefix]_UD_T_[Modelname].shipcfg and [Modelname]_t.xml, and put these files in My Documents\My Games\GC2DarkAvatar\shiptemplates. [Prefix] will be what you set for the prefix in the ship style file, and [Modelname] will be the name of the ship.
Inside the shipcfg file will be very simple. All it needs is:
[HULL]
Version=26
BASE_HULL=[Modelfilename]
That's it. [Modelfilename] should be the name of the model you want to use for this ship (with the .x extension included). Next, the xml file:
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<GC2Ships>
<Ship Name="[Modelname]">
<Component>[Size]Hull0</Component>
<Description>[Description]</Description>
<DisplayName>[in-game name]</DisplayName>
<Model>UD_T_[Modelname]</Model>
<UserDefined>1</UserDefined>
</Ship>
</GC2Ships>
Here [Modelname] can be the same as you used in the filenames, but it really just needs to be unique. [Size] should be Cargo, Tiny, Small, Medium, Large or Huge, this determines what size the ship will act as in the game. [Description] and [in-game name] should be self-explanatory. The model tag just needs to be the filename of the .shipcfg file, minus prefix and extension.
Once you've added those, the ship should be available in-game when you play with the ship style you added. Note that until you have enough ships added to fill all the template slots in the ship style file (you can assign them in-game when customizing an AI), you should not set an AI to use this style.
Of course, the simple thing for you do to would be to just ignore all this and go download the full Babylon 5 mod. It isn't meta-safe, but it already has all the ships set up for use.