squashfs reserve some free space

**Briefly: Is it possible to reserve some free space using mksquashfs?** I'm creating an image of simple livecd based on squashfs filesystem. Everything works fine except that I've got very little "free" space on a root partition. Can I assign more free space with mksquashfs? For now as I workaround I'm mounting ramfs after livecd boot. Another idea will be to create a large file on squashfs, pack it using mksquashfs and delete it after bootup. Is there any "clean" solution?