Sub-districts are added under "Sub-District" category found in Add New > Add Political boundaries, but please be informed that you must have a valid resource like a KML/KMZ file, preferably acquired (legally) from a govt. source, to trace the outline as accurately as possible. You may also be asked to provide the online URL of the resource used so that reviewers can verify, failure to do so may result in denial of your edit on account of being incomplete/scarce/sparse data.