This is not easily done...
You can remove the 'Add Existing...' button from the ribbon and add you own button which could run some JScript to open a custom page with a custom grid that would display the filtered data that you want. Upon selection of the records in the grid you would have to update those records to relate them to the custom contract entity you created. There may be an unsupported way to accomplish this by modifying the query on the 'Add Existing...' form that pops up but I couldn't tell you how to go about that. I think you have to decide if for this little but of functionality if it is worth the amount of time it will take to customize it.
Good Luck!