You can add attributes to shape nodes in Maya, and MtoA will export those extra attributes as user data in the Arnold shape nodes. When the scene is translated to Arnold, attributes are not exported automatically. They must first be added as a Maya 'attribute' to the geometry, where it is visible in the 'Extra Attributes'. On export, MtoA translates them into user attributes on the Arnold geometry.
The following examples show you how to use the 'UserData' nodes: