Yes, you can use either canned or frozen sweet potatoes instead of fresh ones. They don't need to be roasted before being added to a salad. Canned sweet potatoes are already cooked, and frozen sweet potatoes can be added to the salad raw or thawed first. Use a paper towel to pat off excess liquid from the sweet potatoes before adding them to the salad. However, if you want to add more flavor and texture to the salad, you can roast the sweet potatoes before adding them. To roast canned or frozen sweet potatoes, preheat your oven to 375 degrees Fahrenheit. Drain and rinse the canned sweet potatoes or thaw the frozen sweet potatoes. Pat off any excess liquid with a paper towel. Cut the sweet potatoes into cubes or wedges and toss them with olive oil and seasonings of your choice. Spread the sweet potatoes on a baking sheet and roast for 20-25 minutes, or until they are tender and slightly browned. Once the sweet potatoes are roasted, add them to the other salad ingredients and toss to combine. Serve immediately.