You sure can. While leftover mashed potatoes are the easiest, you have many options. My experience with the ready-made mashed potatoes is that they are quite creamy, so you will likely need to add additional flour to the mixture, and possibly omit the egg. If you are starting with raw potatoes and making your own mashed mixture with the intent of creating these crispy potato latkes, I recommend using less butter and cream than usual - go for more of a smashed potato texture than a super creamy, thin texture.