Is there a way to use a custom method for the linkage function?

I can see here that using Z = linkage(X,method,metric) allows for a few different options for computing distance between clusters. However there doesn't seem to be a custom option for the method, only the metric.

Is there a way to override this behavior? When I cluster together two vectors, I would like them to simply add their values together and be treated as a single vector instead of a cluster. My program ultimately has to be compiled as a .NET assembly, if that matters.