Robin Bissell will direct the film and work with Theresa Haggard on the script, while Deadline sources claim Rockwell will re-record Merle's 1960s songs for the soundtrack if he signs on.
Bissell directed Rockwell in last year's "The Best of Enemies".
The Haggard film will detail the singer/songwriter's rise from San Quentin Prison inmate to the country star Kris Kristofferson once called "the greatest artist in American music history".
Haggard was a teen tearaway, who escaped juvenile facilities across California and racked up crime after crime, eventually finding himself at San Quentin for Johnny Cash's famous concert at the prison.
Upon his release, Merle vowed to focus on music as a career and turned his back on crime.
If Rockwell is confirmed for the role, it won't be the first time the actor has portrayed a real person onscreen - he played Bob Fosse in the TV drama "Fosse/Verdon", C.P. Ellis in "The Best of Enemies", Chuck Barris in "Confessions of a Dangerous Mind", and George W. Bush in "Vice (2018)".