  Spectro-temporal modes of light can be exploited for the generation of
high-dimensional Gaussian quantum states. Such states are at the basis of
continuous variable quantum information protocols where they have to support
mode-selective non-Gaussian operations. We develop a general framework for
single-photon addition on multimode states of light via parametric down
conversion processes. We identify the analytical conditions for single-mode and
mode-selective photon addition. We show that spectral mode selectivity can be
achieved in the Type-II collinear down conversion, while single-mode condition
are retrieved for noncollinear Type-I and Type-II processes. Numerical results
are shown for photon addition in parametric down conversion process at
near-infrared and telecommunications wavelengths.
