You sure can re-use resources from TFTD by modding them into the first game, it would take some messing around with the extraSprites ruleset, and the images would automatically be drawn with the first game's palette since the images are indexed, they'd probably look a little weird though. It might be easier to look around for rips of the images to import using extraSprites, and ask around about the battlescape tiles / maps - those have definitely made it into mods on the first game.
The items you recover are actually part of the terrain - for example, the UFO power sources you see on the map count as an item during recovery. One way to add more items to a UFO is to edit the map and just add in more power sources! It'll just not be pretty when they blow after shooting them down.
It's not used in the ruleset for the vanilla game, but you can use the map blocks data in a UFO's definition to place items on the ground that can be picked up during the mission or recovered afterwards - see
here, and the file you'd want to make a copy of for the mod would be openxcom/standard/xcom1/ufos.rul. If you want to see it in action, the mod Piratez makes extensive use of this feature. The issue with this method is that the type of item that is in your stores after recovery for UFO power sources/navigation is not the type that you see in the battlescape - you'd have to make a second item that gets recovered as the first type after the mission in order to use this feature.
