I'm not sure if the Archicad Python API is capable of that. But you could write an external python script and invoke it from an add-on (passing the relevant parameters). It also depends on whether there is a suitable python library for interacting with Sharepoint (otherwise you don't gain anything). Bear in mind that any language use relying on .Net won't work directly in Archicad - otherwise you could do it in C++ too. The primary problem is that MS designs its software to limit interaction in a platform-neutral way. That situation is improving, but I don't believe .Net will every be a level playing field across platforms.
There is a Sharepoint library in Python that can be used to upload a document on a Sharepoint site. The question is : will I be able to call my Python script from ArchiCAD ?
If not, I'd be better trying to do it in C++ directly.