Take this advice with a grain of salt since I don't have matlab in front of me.
In your code,
im_processor should be of type
ij.process.ImageProcessor, so one of these may get your image data into an array format (which matlab should be happy with).
farray = im_processor.getFloatArray()
iarray = im_processor.getIntArray()
If you're using
ImageJ2 then you will indeed want to go from a matlab array to an
ImgLib2 data structure though, and that's a different story. You could try:
my_imglib2_img = ArrayImgs.doubles( my_matlab_array(:), size( my_matlab_array ));
but again, grain of salt, since I can't test this at the moment.
Let me know if this helps, or if I misunderstood something,