Sorry, I think I read your question incorrectly the first time. I thought you were asking if you can perform NAT before the route lookup of the flow.
But looking again I see you want to do normal source NAT before forwarding to the same destination address. That process is outlined on page 6 of these nat examples.
https://kb.juniper.net/library/CUSTOMERSERVICE/technotes/Junos_NAT_Examples.pdf