Can you add a tiny bit of fabric softener to a steam vacuum cleaner for the scent?

Never put any chemicals in your carpet cleaner except for those specifically designed for the machine. Fabric softener would leave a sticky residue in the machine and mess it up, not to mention leave some residue on the carpet that would actually attract more dirt and make the carpet look dirtier in the long run. Instead, spray some febreze on the carpet when you are done.