I'm planning out how to finish a room in the basement.  One of the walls is not uniform and it looks like I'll need to make it about 9" deep to straighten it out.

How would I go about making a doorway where the hamb would be about 9" wide?

Thanks!

Maybe rip down a 12-in piece of pine for the frame and then work the door inside of that

Frame as usual, obviously wider, cover the frame with trim boards after the door is installed.

You trim out the jamb with boards to make it wide enough for the wall.

The pocket screws can be hidden on the outside of the jamb.
Saves the work of having to fill the pockets in.

Have a finish carpenter build a door jam for you.

Not worth the cost.
Just make jamb extensions up from wood of the same thickness, pocket screws on the hidden side and glue.
Done.

If you have access to a biscuit jointer it would make short work of attaching jam extension... But screws will work.

easy way, buy a door kit, and pocket hole some pine to fill out the rest... If your painting it, fill the seem with some wood filler sand, and paint...

If its going to be stained, This is a great chance to learn why buying a pre hung door is always the best solution.. But its not "hard" if you have the tooling.. Rip your lumber to the proper width, mortis for your hinges, and striker plate.. Install jam, check for plumb, and level at the top.. Hang door with only 1 screw in each hinge. Check everything for plumb...Check again... Go upstairs, have a beer or 7, and check again.. Close the door, and check for plumb on the door yet again.. Take a pencil, trace the door where it sits against the jam after its plumb.. Install the stop molding on the door side of your pencil line.. tack molding in place with some 18 gauge brads, top middle and bottom.. close the door and check... If every think looks ok.. stop.. Go the quickest store that sells lottery tickets and buy one.. Now go back home, use a self centering drill bit to pre drill the rest of your hinge holes, install screws and finish tacking in your molding.. This never happens on the first try..

Every time I do this... I think I got this, Ive done this many times before, wont take long... Famous last words...

How much money do you want to spend?

From most expensive to least expensive:

Hire a finish carpenter to hang the door and build jamb extensions.

Buy a custom-depth door. I did this to replace all of the doors in my house with solid-core. I have a house built in 1964 during the transition from plaster to drywall. I have 2' x 4' sheets of 1/2" drywall coated with plaster. Makes the walls 4-1/2" or just a hair over vs. the standard 4".

Buy a standard door and jamb extension kit.

Buy a standard door and make your own jamb extension.

Buy a standard door and wrap the drywall around the corner and finish it in a J-channel along the door jamb. It is a basement after all.

The biggest problem with extension jambs (depending on which way the door swings) is losing your arc swing past ~90° and having the door block 2" of your opening when you are trying to move furniture.

Better to build a custom jamb so you can swing the door wide open.

The jamb stops are usually 1/2 or 3/8 thick.
They reduce the clearance though the jamb.
If the last little bit of the door in the opening is a problem, you can always remove the door.

We had an engineer fired after he measured the width of a door, then made a 19 rack assembly exactly that wide.
It did not fit through any standard door jamb.

The government would not accept the racks.