First of all this note brought me to success after some tries.
What I actually did is (I dont use LVMs at all):
1. cd /usr/ports/sysutils/fusefs-ext4fuse && sudo make install clean
2. sudo /usr/local/etc/rc.d/fusefs onestart
3. sudo ext4fuse /dev/ad0s3 /mnt
4. sudo cp -vRf /mnt/home/user/misc .
where "." is my current location inside my home dir.
/dev/ad0s3 (and /home/user/misc there) was the Arch Linux root (and the only) ext4 partition. Now (being completely migrating to FreeBSD on this desktop PC) I can finally remove this partition. It is no longer needed.
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[user@radicp424 ~]$ uname -a
FreeBSD radicp424.garant.local 9.1-PRERELEASE FreeBSD 9.1-PRERELEASE #0 r243746: Sat Dec 1 18:20:58 FET 2012 root@radicp424.garant.local:/usr/obj/usr/s
FreeBSD radicp424.garant.local 9.1-PRERELEASE FreeBSD 9.1-PRERELEASE #0 r243746: Sat Dec 1 18:20:58 FET 2012 root@radicp424.garant.local:/usr/obj/usr/s
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