Can I use foods that are not on the plan?
Usually, yes. Food lists are teaching tools, not sacred texts. A food can fit if it supports the target: enough protein, appropriate calories, useful carbs or fats, adequate fiber and micronutrients across the day, and good adherence.
When substituting, match the role of the food. Replace a lean protein with another lean protein, a carb source with a similar carb source, and a fat source with a fat source. If a substitute is much higher in calories, fat, or sugar than the original, account for it rather than assuming it is equivalent.