No, that's not true. The certification on the manufactured home is for the manufacturer, certifying that the home complies with the standards in force at the time of manufacturer.

You can make any modification you wish to your home after you take possession. However you will need to get a building permit and AHJ inspection (local building department) for the work, if it involves any changes to the structure, electrical, plumbing, mechanical or energy compliance.

Installing a French Door in-place of a window involves removing the window, its framing and probably some electrical wiring (branch circuits are run along the walls through the studs). The roof will also need to be temporally supported during the construction.

New framing for the door will consist of double (or triple side studs, a new and stronger header, consisting of double 2x6 or 2x8 joists with a plywood filler.

Also, additional concrete piers will need to be installed under the floor, on each side of the door opening. The reason for this is the increased width of the door "moves" more roof load to the edge of the door framing and this additional extra load must be taken directly to earth (the outriggers are not designed to support openings in the sidewalls greater than between 32" to 48: in width - depends on the structural system designed into the home).
Their base should extend to below frost depth, just like the piers under the home. (This is a very important item in changing a window to a door, and its cost must be factored in to the remodeling).

All of this should be done by a reputable local contractor and licensed electrician for your future safety and to keep up the re-saleability of the home. (Trust me on this, if its done wrong, and a future purchaser has a problem, he can come back on you years later. It's a law everywhere that you get a building permit, and most local AHJ will require licensed contractors to do the work. Cheap piece of mind to do the job right the first time).

But to answer your original question, yes, you can make ANY change you wish to your home, as long as you get a building permit. (Well, let me modify that, the only modification you can't do is to remove the frame. It's on permanent by federal law. You CAN remove the wheels and hitch if you want, or put the home on a permanent foundation over a full basement. But very structural part of the frame, outriggers, crossmembers and I-beams, must remain unmodified).

Although you didn't mention it, I should mention that there is only one item you must NEVER loose and that is the federal certification labels. These are the small, red, 2"x4" metal tags installed at the off-hitch end of the home. If you ever decide to reside the home, make absolutely certain that you keep those tags after they remove the factory siding. Without them, it's going to much more difficult, time consuming and expensive to sell the home. (They can NEVER be replaced).
So, if you ever decide to reside the home, after the siding crew finishes, have them permanently reattach them to the same location where they are currently. (I do mean permanent, such as one-way screws or galvanized nails).

Would replacing a sliding glass patio door with French doors require redoing any of the supports or roof?
And thank you for the info about the labels. I never knew that!
Edie