If I buy Sims 4 on steam can I transfer my Origin save games?

@Usless_Vay  Yes, you can use your Origin saves in a Steam-bought game.  You don't even need to "transfer" anything: the two types of installs read the same user folder, as in, the Sims 4 folder in Documents\Electronic Arts.  So a Steam install will pick up your saves and other content without your needing to do anything at all.

For what it's worth, I'd still pay the extra money to stay with Origin and avoid having to run Steam as well.  I see a lot of threads about the Origin-Steam tie-in not working correctly, or not working all the time, or otherwise interfering with what should be a smooth experience.  In the long term, this is a small price to pay for avoiding all that potential aggravation.