Right now, there's nothing that can be done. nVidia's SLI can allow 2+ nVidia chips to work together, & AMD's CrossFireX allows 2+ AMD chips to work together, but both are usually limited to a small family of "related" GPUs to function, & neither one works to SLI/CrossFireX with Intel's integrated GPUs.
However, the rumor is that DirectX 12 will allow for cross-platform processing between different GPUs, possibly even GPUs from different manufacturers. No idea how that would work, or if it would be as efficient as the vendor-specific solutions, but we may just have to wait & see.
Although, to be honest, the Intel graphics are so low on the scale anyway that you'd have to have a low-end discrete GPU for them to even provide any kind of measurable boost anyway.