Both players will move to Dublin on one-year contracts.
Williams, 25, has played Super 12 rugby for Canterbury Crusaders and Auckland Blues, in addition to gaining representative honours with New Zealand Maori.
He also featured for Auckland against the British & Irish Lions over the summer.
Hepworth, meanwhile, was a member of Castleford's Super League squad last summer. The 22-year-old previously played for Leeds Rhinos.
Leinster have also confirmed they will play their three home European Cup group games against Bath, Bourgoin and Glasgow at the Royal Dublin Society Showgrounds.
A 13,500, all-seater arena will be in place for the European fixtures, plus Leinster's Celtic League home clash against Cardiff Blues on October 15.