Applies to: K-Station, KS Rack, KS 4/5
In the "env's" menu on the K Station, you can set the audio input to trigger the envelopes using the 'ENV Aud Trig' option. If you then increase the level of the external input in the mixer section on the synth you will be able to combine the external input and the synth sound.
Alternatively, also in the "env's" menu, you can choose to send the external input straight to the effects section on the synth using the 'ENV In to FX' option.
You can set the external input sensitivity to Mic or Line level in the "Global" menu where you will also find a trim control.