The spaces on a dog cage is by far wider than a ferret cage...they would not be able to be contained...I have two dog crates for our bulldogs...just no way that would work...
I have heard where ones connect cages...that "is" doable...believe it or not...By ferret cage what are you thinking? And have you tried craigslist? It might not be new to you...but it could be an upgraded size...?...
I got my Ferret Nation for $75 off of craigslist with owners reciept it was less than 3 yrs. old...Though I bought Bass Equipment trays...at different times so I ate shipping costs...I wasn't sure if the chinchillas would get along...so I ordered one full tray...So I could seperate them in the ferret nation cage...They got along...so I later ordered the other tray that has the access cut out. $73 I have in them...a total of $148 in my cage...without adding the extra stuff I purchased.
But...it's a thought...they sell chinchilla and ferret cages on craigslist...some ferret cages remind me of the chinchilla cages that are sold on qualitycage.com...but I would say it couldn't hurt to look. But since it sounds as if you still live at home. [could be wrong] if your a minor...you'll want to have your parents contact the seller of anything you would happen upon on craigslist.