alter system appending to a value

Hi all,
as trivial as it sounds, is there a smart way to use ALTER SYSTEM to
append to a value?
Something like: ALTER SYSTEM shared_preloaded_libraries =
current_setting( 'shared_preloaded_libraries' ) || ',foo';

Rationale: I'm using ansible to configure, thru different steps,
PostgreSQL instances and I would like to have every part to append its
configuration on the previous one.

Thanks,
Luca