Combine potatoes and 1 1/2 cups water in a stovetop pressure cooker.
Close cooker securely and place pressure regulator over vent according to manufacturer's instructions. Heat until steam escapes in a steady flow and makes a whistling sound, about 10 minutes. Adjust temperature until regulator is gently rocking. Cook for 10 minutes. Let pressure release naturally according to manufacturer's instructions, 5 to 10 minutes. Unlock lid and remove.
Drain water and mash potatoes. Stir in margarine. Stir in pumpkin, salt, pumpkin pie spice, and basil until combined.