My dandelion wine is like that- it's pretty thin without adding white grape concentrate or raisins. I simply buy the "golden raisins" and chop them up a bit (it's a HUGE PITA!) and add them to secondary and rack the wine on top. It works very well.
You can always add stuff whenever you want. There isn't really a window of time when it's too late to add raisins or grape concentrate as long as you still have some active yeast. If you add in secondary, be careful to watch for foaming up. Since the fermentation will restart, you may not want to top up your carboy until the major foaming has stopped. Then you can top up as usual.
Broken Leg Brewery
Giving beer a leg to stand on since 2006