Can I use sage tea as a herb while cooking
Without a list of ingredients there's no way to know what else is in your sage tea, or what flavors it will impart to a dish. 

If the tea you get from the bag is pleasant then the flavor it imparts to the food should also be good, although sage is a very powerful herb and a whole tea bag of it would likely be far too much. 

My view on this is that while you don't want to waste food you also don't want to ruin it, and the global ecology is not going to collapse because you get rid of a sage tea bag. If you really want to use it rip it open and try it on a small dish first and see what results you get.