Fundamentally, any radar that can be configured to capture coherent data (ie: amplitude and phase rather than just amplitude) can be a SAR. SAR is basically post-hoc digital beamforming to sharpen the image. If you have amplitude and phase information (which you can get if your radar can be configured to output raw quadrature baseband) you can do SAR processing. There's no fancy hardware required, SAR is largely a software problem.

You do also need information about where the sensor was located at each pulse, but it doesn't have to be extremely precise - there are algorithms that do iterative processing to accomplish motion compensation.