I don’t think this feature exists. There’s a place on GitHub for feature suggestions. And Prey is an open source project, so if you have the desire and skills to help implement it, feel free.
From: TimSent: Sunday, July 22, 2012 11:10 AMSubject: [Prey-Security] Multiple SIM cards
I have multiple SIM cards with my Galaxy S3 and keep having to turn Prey off...I know I can keep 'reactivating' the new SIM as I change it but that is a complete faff...Anyway you can 'register' both SIM cards as 'activvated' so you don't have to keep switching Prey to off or going into the app to 're-activate' the SIM?Theft Aware aka Avast Theft Aware now has this 'register SIM' feature...it's really useful, and obviously any thieves are unlikely to have both SIMs and/or put their own in anyway.