Put the port in a small pot over medium heat and warm until steaming but not boiling. Add the raisins and soak until they’re plump and tender, 5 to 10 minutes. When the couscous is ready, fluff it with a fork and let it cool a bit.
Trim the fennel, reserving a few of the feathery fronds for garnish. Cut the bulb in half lengthwise, then slice each half thinly.
Put the raisins and port in a large bowl with 1/4 cup of the vinaigrette and the fennel seeds; whisk to combine. Add the couscous and fennel and toss gently until they’re evenly coated. (At this point you can let the salad sit for up to an hour; in fact, it will benefit if you have the time.) Taste and adjust the seasoning, and add more vinaigrette as you like.