The Digi-Slave 3000 isn't E-TTL compatible and so it can't be used in fully automatic flash exposure. If it can be set to trigger on the 580EX II main plash (rather than the metering pre-flash) you can use both of them together.

I'd put the 580EX II in the hotshoe or on an off camera shoe cord and use the digi-slave 3000 bounced off a diffuser for fill flash. If you don't have a flash meter you'll have to experiment with flash position, lens aperture and Digi-Slave flash power setting to get the degree of fill you need.