I've successfully added refrigerant with a those DIY kits on several vehicles now. My co workers old car stopped blowing cold air and the compressor stopped engaging, I brought it back to life with a can of refrigerant,not a whole can of course. His AC worked fine all summer.
I make sure to only use pure refrigerant, brand doesn't matter, as long as it doesn't have any sealers, seal conditioners, or oils added.
It can be done, but you it can easily be messed up too.