Objects from linked C# code from another (external) project not bundled with Nuget package

Apologies if I have the terminology wrong -  I hope I can make myself clear enough.

I'm trying to build a NuGet package that makes use of local C# source files and those linked from an external location (of the project) - this is because we have multiple projects referencing the same code and unfortunately due to how it is organised I can't use use a Git submodule. While I'm working in the editor the code is working as expected but when I build the NuGet package it seems that none of the code from the linked source files are included.

I'm sure it's a build configuration issue and I've missed a setting but I can't think of what I need to do to get the desired result. As is I'll have to physically copy these source files from the external location into my local project structure which isn't ideal.

Any help is appreciated.