Current Source Transformation
The only difference between a current source and a voltage source is that the current source drives a particular current to the circuit and the voltages build across the components, while a voltage source will supply a voltage that remains constant and the components will pull current based on that voltage. Using Ohm's law and Kirchhoff's equations, we can solve for the voltage or current across each component.

A current source will never be a voltage source, but it will cause a voltage across each component that could be modeled as a voltage source. Use the Thevenin equivalent circuit to model your circuit as a voltage source.

For example, using superposition, you can find a voltage across R5. Then, by shorting the independent voltage sources and current sources, you can find the thevenin resistance at the terminals of R5. This could then be modeled as 







simulate this circuit &ndash; Schematic created using CircuitLab

EDIT

Sorry, my understanding is very conceptual. I now see you were referring to the analytical tool of source transformation. My understanding is that source transformations can only be used in the special case of a current source in parallel with a resistor, or a voltage source in series with a resister, so R3 would violate those requirements.