FEAP does not have this feature. You have two options. (1) Convert the data after it is output with a short post-processing program that you write yourself, or (2) you can modify the element you are using to output the polar values in the tplot stress slots (for example if you are using sld3d1.f, the displacement 3d solid element, then look for where tt( : ) is set
i = 6*(l-1)
do j = 1,6
tt(j+i) = sigp(j,l)
end do ! j
and compute the cylindrical stresses and place them in tt( : ) instead of the Cartessian ones which are in sigp(1:6, l ). Note the ordering in sigp is (11,22,33,12,23,31).