$ [sudo] pip install plist-array

plist="path/to/name.plist"
key="ProgramArguments"


# set
$ set -- value1 value2
$ plist-array set "$plist" "$key" "$@"

# get
$ plist-array get "$plist" "$key"
value1
value2

# add
$ set -- value3 value3
$ plist-array add "$plist" "$key" "$@"
$ plist-array get "$plist" "$key"
value1
value2
value3
value4