Nah, you can mount ext3 and fat32 with little effort.
FAT is the easiest - just use msdosfs as the file system type.
Ext3 will have to be mounted as ext2 , which means the journal will be in a bad state when linux sees it again. It's not dangerous, but you'll have to fsck it.
Actually, let me go back on that somewhat:
If the ext3 file system was created with the wrong inode size, you need a patch to be able to mount it; I don't think that fix has made it into anything but -CURRENT and -STABLE yet. It'll be in 7.2, when that comes around, but for now you might want to read this.
If you are going to create the ext3 partition yourself you can completely avoid this (by making sure it uses 128 byte inodes).
Out of more random interest, what ext3 partitions will you be mounting on it? External drive formatted for use with linux, or?
my old server is linux with local disks formatted as ext3 and it have external usb disks formatted as fat32, I only need to transfer from the linux disks to the new freebsd raid, and plug the usb disks to the freebsd server as well.