renluop
It's a long time since I've used it and I'm not sure I ever attempted to insert anything just replace a faulty bit of hex code. However I have found a work around since Eric's suggestion wouldn't work for me. If you go to the end of the file you want to edit it is quite simple to type in the additional code you want to tag on. Just make sure you are not in Read Only mode (about half way down in the Edit Menu tab). Follow this with the FF D9 bytes you want to add, high-lite them with the mouse (turns yellow), cut and paste. When you select cut a box pops up, just select OK. Position the cursor at the beginning of your file and select paste . Another box pops up and select insert and OK.
Tony
Edit. Posted anyway but I see Geep beat me to it. His method is neater than mine but both will work. (for the edit, not sure if it will fix your jpeg!)