If you already own The Sims 3 or any of the Expansion/Stuff Packs, and wish to register them on your Origin account, please follow the steps below. Registering The Sims 3 game and Expansions/Stuff Packs to your Origin account via the Sims 3 website
After you've checked that your game is in your My Games library, head to to redeem your bonus content codes or download your registration rewards. The Sims 3, expansions, and Stuff Packs can all be redeemed through Origin or, but once redeemed do not need to be re-registered in the other. Note, codes for bonus content and The Sims 3 Store content are redeemed through, not through Origin.
To access them, right-click on the Sims 3 icon and choose view game details.
You can then re-enable the internet and Origin will remain offline.
It will notify you that you are offline and ask for you to log in (even though you aren't online).
You can temporarily disable your internet (easy on a laptop, not so sure about a desktop) and launch Origin. MrsFlynn's instructions are perfect for going offline, but there's a way to launch Origin in off-line mode to start if that's your preference. If you purchased/installed through Origin, I have no idea if you can by-pass it. I use TS3.exe to by-passes the launcher, or I use Sims3Launcher.exe to see the launcher first. I don't use the expansion directory my files are in: \\\Program Files (x86)\Electronic Arts\The Sims 3\Game\Bin\. However, I purchased and installed them prior to Origin (but I am able to play through Origin if I want). I have two files that I use to play TS3 without Origin.