Create new function for Arcpy

Hello,

As a result for my graduation paper I am trying to create a new toolbox for ArcGIS using Python scripting.
The problem is I am stuck with my code because none of the existing functions in Arcpy does what I need to do.
So my question is, is it possible to create a new function in Arcpy or this is restricted to ESRI developers?

Another way to solve this problem would be implement some changes in the tool Cost Distance from Spatial Analyst.
So my other question is, do I have access to the coding of the native tools from ArcGIS. And if I have, can I change it to achieve my
goal? Or this is also restricted?

Thanks,

Gabriel