I think you might be able to use resize2fs to do this ... from the man page it states:
The resize2fs program does not manipulate the size of partitions. If you wish to enlarge a filesystem, you must first make sure you can expand the size of the underlying partition first. This can be done using fdisk(8) by deleting the partition and recreating it with a larger size or using lvextend(8), if you're using the logical volume manager lvm(8). When recreating the partition, make sure you create it with the same starting disk cylinder as before! Otherwise, the resize operation will certainly not work, and you may lose your entire filesystem. After running fdisk(8), run resize2fs to resize the ext2 filesystem to use all of the space in the newly enlarged partition.
YMMV, no warranty implied, backup your data to be on the safe side!