The Starz drama, currently airing its second season, was picked up for a third run in November last year.
Lasance - known for his role in Australian soap Home and Away and mini-series Cloudstreet - will appear in the new episodes as a young Caesar.
In the early days of his ascent toward ruling the Roman empire, Caesar will seek to crush the slave rebellion led by Spartacus (Liam McIntyre).
The Wolfman star Simon Merrells has also been cast as the power-hungry Marcus Licinius Crassus, the richest man in the Roman republic.
The second season of Spartacus has so far averaged 3.1m viewers for Starz.
The series continues on Fridays at 10/9c. In the UK, the show airs on Mondays at 10pm on Sky1.
> Spartacus Liam McIntyre Q&A: 'It's a big mantle to take on'
> Lucy Lawless Spartacus: Vengeance Q&A: 'This season is enormous'
Watch a preview for the next Spartacus below: