pinki - Yes you can freeze mashed potato. Make up your mash potato as you normally would, use a potato scoop & scoop several blobs on to a baking tray, put in the freezer with a bag over them to completely cover them ( stops ice crystals forming ) Once they are frozen, bang the tray gently and pack in to bags or freezer containers. If you make a massive batch it saves a lot of time!
psychick - Defrost/reheat frozen mash in the microwave in a ceramic or microwave bowl, make sure you place a ceramic or microwable lid on top while cooking, this will trap the moisture and stop the potatoes drying out.