When Pets came out, EA released "TS3 Plus Pets" which has the base game with the EP.

I've got about $25 right now and the EP by itself is still selling for $35 in the US. But I've noticed TS3 Plus Pets sells for a lot cheaper on ebay- like closer to $20.

So, obviously I already have the base game, but I was wondering what would happen if I got the bundled version and tried to add it to my game. Could I just delete the new TS3 base game folder that gets installed and keep the Pets stuff, or would that not work?

My suggestion would be to make backups of your custom stuff, saves, etc. and just roll with the bundle. Why would you want to install that anywhere else? Just install the thing, move back your custom stuff and you're fine.