Summary
You can use Metz grip-type flash units with their SCA adapter to provide automatic fill-in flash with the Phase One Camera System. This is explained on pages 39-43 of the Phase One 645 AF Camera Manual.
In-Depth
Other options include the following: Canon has a flash unit for their EOS cameras, the 580EX II, for example, works well with the Phase One System. Amongst the features it includes a Custom Function called Auto Aperture. When it is set to this mode, you can key in the aperture you are using and the ISO on the flash unit. It will then measure the light from the subject to give correct exposure. The Canon flash allows you to input any aperture in 1/3rd increments. As such, if you want the fill flash at one stop under the daylight, just key in a stop below ambient exposure.
This system, and ones like it, work extremely well with the Phase One 645 Camera System.