Basically, yes. When you go to load game in the main menu, select your limit cut save file by pressing square to boot into the main game. Collect whatever you want, then save your game and exit out to the title screen. When you reload (with your regular select button this time) you should be back into the limit cut content but with the extra items/whatever.
Considering the commonness of this question I really think they should update the menu or something with at least a one time pop up so that people can find the option easier or at least know that this is okay and won't erase your progress or anything.