Yes you could do that. However, a lot of juices be they DIY or from a vendor need time to steep. That brings out the flavor. Sometimes a more intense flavor, and sometimes the flavor changes and you taste different essences of the juice. Adding a concentrated flavor to already mixed juice could have unintended consequences. More flavoring could kill the flavor altogether.
Here's a method you can use for all your favorite juices. Steep it, put it in a cup of hot ( not boiling ) water. When it gets to room temperature remove and take off the cap. Let it stay open in a cool dark space for at least 8 hours. Put the cap back on and let it sit for 2 days. If you can, shake it at least once a day. Try the juice a little bit each day in the PV your going to use for that juice. It could take up to a month with some juices, to ripen to their full flavor. When it taste the way you want, you'll have a method for that juice you can follow with each new batch.