Though overlooked in favour of Sergey Sirotkin for a race berth, Kubica was named as Williams' test and development driver.
It will see the 33-year-old take the wheel of the FW41 in testing, and during a number of Friday practice sessions at events during the season.
Alongside that role, Kubica will this week join Manor as the British squad begins a three-day test at the Aragon circuit in Spain.
Though there is currently no deal in place between Kubica and Manor, the test could lead to the grand prix winner joining the team in the WEC this season.
Should he ink a deal with Manor, it would reunite him with the team he raced with at the 2004 Macau Grand Prix, where he was team-mate with Lewis Hamilton.
It's not the first time Kubica has looked at a WEC drive, having tested with ByKolles ahead of last season before electing not to compete with the team.
Manor is planning a two-car LMP1 program with Oliver Rowland and Charlie Robertson already confirmed for the squad.
The opening round of the 2018/19 World Endurance Championship takes place at Spa-Francorchamps, Belgium, on May 5.