The Office property is single-valued in AD and thus it is not possible to specify multiple values for it. As a solution, you can use one of Adaxes custom multi-valued attributes (e.g. CustomAttributeTextMultiValue1) on all the Web Interface forms and configure the attribute to be displayed as drop-down.
As Adaxes custom attributes are not available in Active Directory, you can set values of the multi-valued attribute to the Office property (e.g. add values separated by commas) using Business Rules triggering After creating a user and After updating a user. The update should be performed using a script. If this is what you need, but you have issues configuring the workflow, we will provide you with detailed instructions and the script.