According to Sportitalia, the player has agreed personal terms on a contract worth €3m per season plus bonuses.
The two clubs are still working out the finer details of the move, but it is believed they will eventually reach an agreement for €6m.
The 31-year-old striker only joined Udinese from Watford in January 2019, initially on loan and then on a permanent transfer.
He is well-travelled, as his previous clubs include Roma, Fulham, Parma, Spezia and RSC Anderlecht.
Okaka scored eight goals in 33 Serie A appearances for Udinese last term.