Calculating Thevenin equivalent circuit with series current source
That could only be replaced by an infinite voltage source.

Current sources operate on the basis that they will output their current no matter the load. When you leave it open circuit your open circuit voltage would be infinity. IE, this circuit cannot exist.

The standard replacement will be a current source with a parallel resistance to a voltage source with series resistance. As shown in this wikipedia image.

http://en.wikipedia.org/wiki/Th%C3%A9venin&#39;s_theorem">