Well there's no such thing as an AIX LPAR, operating systems like Linux and AIX run inside an LPAR on pSeries, zSeries hardware etc.
But anyway. If it's just ext3 then you can run resize2fs to extend a filesystem, but more fundamentally it's not the filesystems that matter, but the partitions. That's what is sitting between the LPAR disk allocation and the filesystems. tools like parted are useful for moving around disk partitions, and I would assume that on an LPAR it'd be no different to a normal disk. If you have LVM involved as well then you have the complicated of managing those logical partitions.