If you want to use external partitioning you have to export the whole device via USB.
When you are running Harmattan and connect USB in memory card mode, you are just exporting one partition (by default /dev/mmcblk0p1 which is your MyDocs)
There is no way you can partition the device as the parts that contain partition infomation are not exported.

When you boot into MOSLO or ubiboot maintanance kernel, your whole /dev/mmcblk0 is exported, also the partition tables. Hence, you can modify everything on the drive.

The thing is, you do not have to flash anything permanently if you do not want to, it is enough to jumpstart the device with a maintanance kernel (sudo flasher -k <repairkernel> -l -b)

Note that now, after you do this, your device will be in Open Mode regardless of what mode it was on before.
If you were in Closed Mode you will encounter some weirdness caused by the device aegis keys no longer matching the device mode; for example you will not be able to store passwords before you regenerate the password storage with new keys.
Also you will need to patch your loader or you will experience application shutdown when installing new applications, for example.

If you are in Closed Mode, I will always recommend that you do not enter Open Mode accidentally, but rather back up your device and enter Open Mode via the back-to-back flashing method.
When you do that, it will save you tons of grief later...