Can I mix two or more spot colors? Or Combine a Spot-color Tint with a CMYK Tone?

For Example combine a HKS-spot-Color Tint with Black to get an approximation of the PACKAGE HKS 3000+ tones?

No, at the moment you can't create a new color by mixing together other colors (eg. a custom DeviceN colorspace in PDF). We do plan to support this in future.