Is there some facility in the LaTeX3 kernel to modify existing commands like
expl3 language is useful for both programming with existing LaTeX2e code and creating new code, and so there are some facilities which are '2e-facing'. However, patching existing commands is not an area we wish to encourage: it is much better if interfaces are provided for altering code flows, etc. Patching for LaTeX2e can as you note be achieved using
etoolbox, whilst it should not be used for
expl3 code at all (rather discussions should be undertaken to provide appropriate interfaces).
appto are more-or-less the same at
\tl_put_right:Nn, but there are no equivalents of