IMO a migration path between kernels would be expecting too much out of an OSS project, but maybe I just have lower expectations for these things.
The true irony to me is that they probably could support it if they were going the other direction, since FreeBSD does have a Linux-compatible personality. Actually, you might still be able to leverage that going the other way... release X is based on FreeBSD, release X+1 uses the FreeBSD kernel but migrates to Void Linux userland complete with actual Linux ELF binaries and can still run FreeBSD binaries, release X+2 switches to just running the Void Linux binaries on a Linux kernel, and only moving one release at a time is supported. It'd just be an insane amount of work.