FFF has a round-about way of doing that using the
replace_metadata split feature:
Code:
[www.fimfiction.net]
add_to_replace_metadata:
characters=>^(.*)$=>\1\,Human&&genre==Human
This says to replace any and all
characters entries with themself plus add Human to the list, but only when
genre contains Human. Duplicates are removed automatically.
See
wiki page.
Note that this will only work when there is already something in
characters.