I have two exterior door that have wood rot from the threshold up about six inches. Is there a way to make repairs without buying a complete door and jam?

Is the rot on the hinge side? That side carries some weight of the door and makes it more difficult to cut and install a dutchman, but that would be a fix for you to consider.

Thank you, ballengerb1. Both sides of the jam are rotten.

More pictures;
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It can be done but by the time you get through you will wish that you had just bought a new pre-hung door. Besides that you have rot in the sill under the threshold and the siding to the left of the door. Door needs to come out to fix those things and you need to caulk between the brick mold and the siding. Deck board appears to be tight up against the siding. Need some space for drainage.

I agree with Harold as usual. I am guessing no gutter above this door, that would help down the road too.

If I may offer a bit of bad experience...

You really should pull up the bottom layer of siding and check the sole plate. Given that you don't have a gutter... and given that you've had that much rot, you may have serious issue with the sole plate. I really recommend checking it. The sole plate replacement isn't cheap (ran me about $800 for a 12 foot section here in Texas), but replacing it is MUCH cheaper than doing the repairs that will be needed if it rots through and the wall sags. If that happens, you're looking at drywall work, roof repair AND repairing the sole plate.

cpfrost, you posted about a rim joist being rotten as well in another post. I can see why.
I am looking at your door and thinking that that even if you could repair it, you should really take off those connections on your deck and replace the rim joist and sill plate. I saw in one of your pics from the other posts some sag already. If you do this now, you will prevent further problems later.

Get that deck (like they say above) off the side of the house and put in some flashing too.
Then replace your siding piece and a new door frame.

Also looks like someone covered up an area with some boards under your deck just below where it connects. I wouldn't say a word if I haven't done this type of work, and seen this problem before. Do it right, and you won't have to worry about it.