most pens like this can be added too. i have one like that and i to had thought about getting two more and connecting them so i can make a big one in the backyard for morgan to play in. Then i just bought 3 of these
http://www.petco.com/product/100147/...le-Medium.aspx in the small variety (really light weight cable) and connected them to the patio. Much cheaper and she gets to run around without getting out of the yard. She never gets caught up in it or choked by running to the end and stopping short.