Starting for the 4.4 release, custom prefixes can be added in the administration UI. These prefixes can be used by non-admin users when integrating CSV files (for specifying URIs, for instance)

Registering a prefix (admin-only)

You can head to Prefixes in the user tab. There, you will be able to create and delete custom prefixes.

Creating a custom prefix

Simply fill out the desired prefix (ex: wikibase), and namespace: (ex: Users will be able to fill out data using the wikibase:XXX format.