In flow networks, may source/sink have incoming/outgoing edges? I was wondering. May the source and sink have in-out going edges in a flow-network, and if so - does Ford-Fulkerson and the max-flow min-cut theorem apply ?

Flow-networks are always pictures with no edges entering source, and no edges leaving sink.

I've tried to search the web for an answer, but did not come across an answer i fully understand. Also, I've yet to see a flow network pictured with these edges from source/sink.

Could I transform an undirected graph into a flow-network ? This network will have edges going into source and edges leaving sink ?? This hypothesis is the reason for my question.
Yes, it's possible. 

My source is 'Graph Theory and its Applications', 2nd ed, by Gross &amp; Yellen, page 540, question 13.1.8: 


  Some authors define an s-t network such that the source s must have indegree 0 and t must have outdegree 0. Describe how a network whose source and sink do not satisfy these additional requirements can be transformed by graphical operations into one whose source and sink do meet such requirements. 


However, it doesn't give the construction, I can only think that it would be by adding a supersource and a supersink, as you'd usually do with multiple (though standard) sources and sinks, to 'soak up the leakage'. I am not sure though.