You don't need to wait for complete fermentation to pitch a starter. In fact, it's normal a good idea not to wait that long.
In a starter you want to grow healthy cells, don't worry about fermenting the wort. Most of the cells are actually generated within the first 24-48 hours. This is the end of the exponential growth phase. When the gravity is about half of the starting gravity nearly all of the cells have been produced.
For details see here: