If you use testingcheatsenabled true, you can then go to the Sims house, Shift click on the floor and select buy or build on this lot. You can then buy and place the object(s) you want or even recolor their entire home. It will use your funds so make sure you've got plenty of Simoleons before you do it. I used to have my very wealthy Sims do this for poverty stricken townies a lot. They never seemed to notice the difference though, which was rather disappointing. I was looking forward to the hand clapping happy dance.