Many neurosurg programs include PGY-1 as part of their training. So you'd just upload your neurosurg PS and letters into ERAS, and rank the prelim surgery year at the school where you matched as your only program. And I',m sure you could use the letters and PS, or a modified version, to apply for a separate prelim year as well.
It's unclear from the SFmatch site whether or not there are some programs where you don't have to go through NRMP for the prelim year. It seems to say that, but my dean's office seems to think we have to go through NRMP no matter what.