Hi there and welcome to the community!
I don’t believe Battlestations: Pacific is a modifiable game with Horizon given the list at the top of the program. As for adding PC mods to Xbox games, the best you can do is find modded game saves online (trusted source) then copy them to a USB then open them in Horizon to match the identifiers of your console and profile ID then copy/move them to your console to see if they are legit.
Here is a link to the Horizon tutorial thread: Horizon | The Basics.
Also, Horizon does allow Gold modifications, however, they are at your own risk since you can be banned for not being careful.
If you have any other questions, please post back here.
Hope this helps!