If you talk to people at your program they can give you guidance on how to accomplish this - it's pretty common for doctoral level school psychs to do PP on nights/weekends, but it means you'll need to accrue some extra experience in a non-school setting which can require some more time/planning than those who stick to just working in schools. Also the majority of SPs work at the elementary level, so you're highly likely to be able to work there.
As for pre-k, preschool evaluations are frequently done by independent agencies and/or private practice evaluators, so I would suggest you start doing some research on this as a possible career as well. This is an area of high need in some places. You could also look into serving as the CPSE chair in your district once you're working, though that doesn't really mean you'd have much direct contact with pre-k students, just help guide decisions for the kinds of services (PT/OT/Speech) they'd receive before entering elementary school by collaborating with their parents, teachers and other service providers.