Work a round I used in a project was to set a default value in the FIM client, below the steps to configure:
You can set a default value in the FIM client:
On the server running the User Profile Synchronization Service, open miisclient.exe (Located at Drive:\Program Files\Microsoft Office Servers\14.0\Synchronization Service\UIShell)
Click on the “Management Agents” tab
There should be a management agent in the list that represents your synchronization connection. For Active Directory synchronization connections, the management agent will have the name MOSSAD-YourSynchConnectionName . Double click on the management agent that represents your synchronization connection
Click on “Configure Attribute Flow”
In this screen you see all properties and it is possible to bind a 'constant' to a user profile property.
This work around will work, but it is important to note it will leave you with a User Profile Service which is in an unsupported state. According to Spence Harbar the following applies
Forefront Identity Manager
A bundled version of FIM that includes two
Windows Services, and associated configuration and data. It is not
supported to use the FIM client tool but this can be useful for
viewing progress and identifying errors. The two FIM services are
configured by the User Profile Synchronization Service SharePoint
Blog post by Spence
In other words, leave the configuration of FIM to SharePoint, anything you cannot configure through the SharePoint UI or object model is not a supported config change for FIM.