1. boot into linux in order to copy/write a file from an ext4 partition to a Fat32 partition.
then
2. reboot into FreeBSD in order to copy/write that file from the Fat32 partition to UFS/ZFS.
So .. ya .. I have pushed one-to-many files using FATA32 between FreeBSD and Linux (or) Linux and FreeBSD in the past..
(BUT) you can run into MAX SIZE file issues with FAT32 so it's not a slam dunk. In fact I had to abort (one of my) copies because something I was moving was too large for FAT32 and FAT32 did not like it. It wasn't a deal breaker because I could (also) push the files FAT32 was unhappy with over the network (aka sftp, SAMBA/CIFS, etc) between the hosts.
For full on backups I would NOT recommend doing this.