If you have the model of the seat it will be fairly straightforward.
Assuming you do, a good place to start is to make a copy of the surfaces of the seat with 'knit surface' or 'offset surface' feature and then use the 'thicken' feature to add the cushion material thickness to the surface copies.
There may be some clean up or simplification work to do between those steps, such as getting rid of the little trenches between the raised padded areas, and trimming your surfaces to the desired location for the cushion to end, etc. etc.
If you don't have the model or at least dimensioned drawings of the seat it's going to be a pain in the butt (har har?).
You might try to get a 3d scan of the seat to at least get some geometry to work with.
Either way, it's going to be a great exercise in the surface command set.
"Either way, it's going to be a great exercise in the surface command set" I like this comment.
What Robert says is correct.
I think you'll also need to examine why the seat is being modeled.
If the goal is to simply make a model that looks like a seat, then the challenge will mostly be with the surfacing and curve tools.
If the goal is the design a seat cushion so it can be used to create tooling for mass productions, then you have a more difficult task. Now the model needs to be made for both production, and comfort/use. What proportions best fit a human? How much of a radius should be behind the knees? ...etc.