date arithmetic with columns

I have two columns in two distinct tables, one is the starting time of
an event, timestamp without time zone. Data is the utc datetime (for
sorting across time zones), the other is the number of minutes to add.

I am migrating from Firebird. One of the queries uses the dateadd
function to build a local starting time thus:

SELECT CAST(DATEADD(tz.diffmins MINUTE TO r.utc) AS TIME) AS ts
FROM races r JOIN tracks t ON t.trk = r.trk
JOIN timezones tz on tz.state = t.state....

The equivalent postgres would be along the lines of

SELECT r.utc + INTERVAL '480 minutes'

How can I substitute the hard-coded 480 for the tz.diffmins?