I spoke with Citypass and asked about your question.
You may at US purchase a SoCal Citypass, I was told this may be a good idea too because Universal may offer there own incentive to do so.
But if you can upgrade a US ticket to a SocalCity pass is upto Universal, so you would have to ask them, maybe ahead of time or while getting your US ticket.
I know this is a definite answer that you wanted, but I like that who I spoke to at Citypass suggested buying the citypass while at US because US might have a deal on it.