What's the gravity of the porter? Ideally, starters are closer to the 1.040 range, but to answer your question, it certainly will work in terms of creating more yeast. I'm really not sure what effect using higher gravity wort does to a starter and haven't tried it myself, but if you were in a pinch and needed to make a starter I wouldn't hesitate to use it. But if you're just saving it for the sake of saving it for a starter sometime down the road, I dunno, probably just as easy to make one fresh with some DME.