Referencing Unity data structures in my external library

I am creating an external client/server application that has the ability to send custom protocol messages that are user-defined.

When using my library in Unity, I had to create a script in Unity that essentially wraps my send/receive functions to take in Unity data structures such as Vector3, Quaternion, etc.

I won't get into too many specifics, but on the backend I essentially just send the data across and make sure that the byte sizes all match up (among some other things).

I was wondering - is there a way to reference Unity data structures in my external application? This would be awesome, because it could support Unity as a standalone application instead of having to have a script in Unity that wraps the .dll functions and behavior for ease of use.

Example: currently for using Vector3, I have a class in my external application called Float3 which I just use to transform data to and from Vector3 in Unity, like so:

 public static Vector3 FromFloat3(Float3 f3)
 {
     return new Vector3(f3.x, f3.y, f3.z);
 }

Its not the end of the world, but it would be much better to have this all built in to my application so I don't have to expose a Unity script...