Yes, you can. The only thing that will stop a SCOBY from performing the second fermentation is the alcohol content of the Kombucha.  The yeast feeds on sugar, so if there's no sugar in your starter liquid (thus the Kombucha), then it will stop. Step 1: Get a bottle of kombucha. You can get a bottle from the grocery store, or you could save one from an old batch. Just make sure that it is still sealed tight if you are using an old bottle.  If the seal is broken, bacteria may have gotten into the kombucha and started fermenting. If you are using an old bottle, check for carbonation or a change in color before proceeding with this tutorial. Step 2: Add sweet tea to the bottle of kombucha Open up your bottle of kombucha and add some sweet tea. If you're using a brand new bottle, heat up the tea before adding it. The sugars in kombucha feed off the tea, so you need to add them for the kombucha to start fermenting. Step 3: Wait and hope! This is the part where you have to be patient. Leave your sealed bottle of kombucha and sweet tea at room temperature for a few days or weeks—however long it takes for the kombucha to ferment.  You'll see a new film of SCOBY forming on top when it's done. Step 4: Remove the SCOBY and add the mixture to a larger bottle! Once you've got an excellent film of SCOBY on top, it's time to remove the culture. Pour out the remainder of the liquid in your bottle and make a new batch of sweet tea.  You'll need to add a little salt to it this time—the chemicals from the salt will help your new culture grow.  After a couple of days, you'll see a new film forming on the top. Once it gets going, pour that into a larger bottle and add the mixture you already tried. Bang!  You've made kombucha from a bottle of store-bought kombucha.