Yes, you can Crossfire a discrete GPU with the IGP of AMD APU. The GPU in AMD Trinity processors are based on VLIW4 architecture (not GCN even their nomenclature is HD7000), same architecture found in AMD HD 6900 series. So with an AMD A10 and A8 trinity processors, you can Crossfire any 5000 or 6000 series cards up to HD 6670. This thing is known as Hybrid Crossfire.
But remember, with the current Drivers, in only works with DirectX 11 titles and the performance is not consistence in couple of cases. The current drivers need some more fine tuning.