First: You do not need extras-devel or extras-testing repositories enabled to install rootsh, rootsh package is in extras repository, which is enabled in PR1.3 firmware by default.
This is simply refresh the package list in HAM, which it does if you choose to install anything for the first time, and install rootsh.

Second: You could prepare your custom rootfs image by using backupmenu and build that according to this post http://talk.maemo.org/showthread.php?t=63975, but you will need a PC running Linux. Either installed or Live-CD will work fine.
Note, please, this is no complete Firmware image. you are creating, only rootfs.
Basically, after preparing your custom rootfs image, there is an additional flash command necessary after the firmware flash to put your custom rootfs "over" the flashed rootfs from the firmware.