Appending key-value to JSONB tree

I have a "user" document with a key "tokens" and I would like to write a
stored procedure for adding new token key-value pairs to the "tokens" part
of the tree without removing the old values. I have figured out how to
replace the existing value in the "tokens", but I cannot seem to wrap my
head around appending a new key-value pair. Could someone suggest an
approach (using PostgreSQL 9.5 BTW)...

Here's my existing stored proc:

CREATE OR REPLACE FUNCTION set_user_token(email TEXT, expiration INTERVAL)
RETURNS TEXT AS $$

WITH newtoken AS (

SELECT

jsonb_build_object(random_string(32), (now()+$2)) token

),
updated AS (

SELECT

jsonb_set(data::jsonb, '{"tokens"}', (SELECT token FROM newtoken)) newdata

FROM

users

WHERE

email=$1

),
updatecomplete AS (

UPDATE

cc_users

SET

data=(SELECT newdata FROM updated)

WHERE

email=$1

)
SELECT jsonb_pretty(token) FROM newtoken $$

LANGUAGE SQL;

Thanks in advance!!!

Deven Phillips