Change RSS Feed for wordpress blog

I need my RSS feed from wordpress blog to be changed. The title field needs to be title + ' - ' + customfield(discount) In the rss feed below you will see 1507 & 50% off your bill 1507 Becomes 1507 - 50% off your bill EG. Currently 1507 [login to view URL] [login to view URL] Mon, 15 Jul 2013 12:23:24 +0000 admin [login to view URL] 50% off your bill [login to view URL] 0 After you make the change 1507 - 50% off your bill [login to view URL] [login to view URL] Mon, 15 Jul 2013 12:23:24 +0000 admin [login to view URL] 50% off your bill [login to view URL] 0 I hope this make sense. Should be a simple change for someone who knows there way around wordpress. Many thanks David