I have checked the Relative Polarimetric Calibration steps in the given reference for ALOS and would like to confirm if all of them are already implemented at the complex radiometric calibration in SNAP:

Although I can not assign the new bands as ‘real’ or ‘imaginary’ in order to run a decomposition or other polarimetric operator.
The SNAP does not recognize them as Scattering components.

Did you set the ‘unit’ to real and imaginary?
You could also try not making the new band virtual and removing the original i q bands such that there is only the i q FR bands in the product.

dear @gusortiz: How did you calculate the FR_angle variable?
I looked it up in Annex C in the PDF you mentioned but I’m not sure about the notation in SNAP.