Automatic update via superoffice API

Hi,

we are making some autoconnections and first out will be to update 1 field for users in superoffice based on an external list. I know how to limit for whiom the update shoulc be but at first glanse it seems I need to update all fields (otherwise it writes NULL value). This is a bit odd for me and I would like to restrict it only to update a certain field - is this possible? 

Thank you!