結論から書くと、C#でC++のdllを動かせれば、Unityでも同様に動きます。

・C++dll
extern "C" __declspec(dllexport) float func()
{
    return 1.0f;
}

・C#でC++dllを動かす
using System.Runtime.InteropServices;

namespace dllTestSharp
{
    class Program
    {
        [System.Runtime.InteropServices.DllImport("dllTest.dll")]
        static extern float func();

        static void Main(string[] args)
        {
            Console.WriteLine(func().ToString());
        }
    }
}

・UnityでC++dllを動かす(重力を設定するスクリプトの例)
using System.Runtime.InteropServices;

public class dllTest : MonoBehaviour {

    [System.Runtime.InteropServices.DllImport("dllTest.dll")]
    static extern float func();

    // Use this for initialization
    void Start () {
    }
   
    // Update is called once per frame
    void Update () {
        Physics.gravity = new Vector3(0.0f, func(), 0.0f);
    }
}