It is perfectly possible to buy a 24-hours extension ticket (zone upgrade) in addition to your existing 24hrs-ticket. The 24-hours tickets are available for 1–2 zones, and up to 7 zones; see here: Zone upgrade in the ZVV region – ZVV
In your case, a 1-2 zones 24-hours extension ticket is sufficient if you only want to travel to Zurich city. However, a 9-o-clock pass for Zurich city, plus an extension ticket for 4 zones would allow you to travel in the entire ZVV network. You might want to compare prices, depending on where you would like to go.
Please let us know should you have more queries.