Call a custom Class from PhotonNetwork.cs

Hey,

In my game I need to make a small change in PhotonNetwork.cs.
From withing this PhotonNetwork Class I need to call a Class of my own.

But It seems I can't access it. Probably because it's in a different assembly.
I tried to put my Class in a namespace and included it in the PhotonNetwork.cs Class.
But it doesn't recognises it.

Does somebody had a similar issue and possible solution?

Regards,