If on the dining plan, when using a table service credit you only pay the tip. I don't think you can covert table service credits into snack credits. Even if you could, it wouldn't be worth it.
You can use a TS credit for a QS meal, but not for snack credits. In order to use a TS credit for QS, you must first use up all of your QS credits. Only then will they pull from the QS balance. Alcohol, tips, and appetizers are examples of things typically not covered on the regular DDP.
You cannot trade a table service meal for a snack. It used to be that some of the CS would allow you to trade a CS credit for anywhere from one to three snacks. It wasn't something official, it's just something some of the CS did. Apparently some of them still do this, but many won't do it. You cannot "convert" either TS or CS credits to actual snack credits.
If you are paying for the dining plan, trading a TS credit for a snack would be an almost complete waste of the credit's value - just on the other side of not using the credit at all.
It's usually suggested that if you're not doing free dining, and you don't think you will be using all your TS credits, consider either not getting a dining plan at all, or get the QS plan and pay for a couple of extra TS meals.