The phrase "sweeten kitty" usually refers to making a deal more attractive or appealing by offering some sort of incentive. There are many synonyms that can be used to convey the same idea, such as "sweeten the pot," "sweeten the deal," "sweeten the offer," "sweeten the arrangement," and "sweeten the agreement." All of these phrases imply that something is being added to make a proposal more desirable or advantageous. Depending on the context, other words like "improve," "augment," "enhance," "boost," or "amplify" may also be appropriate to describe what is being done to make a deal more attractive.