You certainly could add additional juice to secondary.
But... It will bring a lot of sugars, unless you want to inhibit the yeast prior or pasteurize it could re-start a ferment if you don't. If you wanted to bottle carbonate you could use the sugars in the juice instead of priming sugar if your cider is at 1.000 or there about. You would have to understand how much sugar is in the juice and calculate the correct amount of juice to add to get the CO2 volume you wanted. It might not be enough flavor for what your looking for though.
On a side note and somewhat unrelated (Off topic)
- I have used other juices in primary with my apple juice and have been pretty happy with the results.
- Oranges in secondary do work pretty well. I have found you need a relatively large amount (1.5 - 3 pounds per gallon depending upon the flavor you are looking for.) In the case of citrus fruit i always peel them, remove all or as much of the pith as possible. Cut the sections in half to expose the fruit and freeze it then and just dump them in frozen. Wait 10 - 14 days and remove. (Pretty much every fruit gives up almost all of its flavor in 2 weeks or so.)
