Instructions
Select "Willy Builder" from the "Viva Pinata" shop menu.
Purchase the Mystery House item. The Mystery House costs 1,980 coins.
Place the Mystery Home near your Swanana and Rashberry.
Select "Ivor Bargain" from the shop menu.
Purchase two pieces of Joy Candy. Joy Candy costs 83 coins per candy.
Place one piece of Joy Candy next to your Swanana and Pigxie.
Select the Swanana. Place your cursor over the Joy Candy and select "Direct" to make the Swanana eat the Joy Candy. Do the same for the Pigxie. After eating the Joy Candy, both Swanana and Pigxie enter the Mystery House to romance, creating an egg that appears outside the house once the romance cutscene has ended.
Wait a few seconds in your garden. The egg hatches and a Pigxie is born. Repeat the process to add additional Pigxies to your garden.