I want to use the Motifs A/D Inputs to bring the signal from another keyboard into my Motif. This keyboard is played over MIDI from my Motif.
When I turn down the Motifs Volume using the main volume slider the signal from the other keyboard also disappears.
Is there any setting for voice and performance mode in which the signal from the A/D Inputs is not effected by the volume slider and just passes the Motif?

No. The Main Volume slider is just that. It controls the main volume output of the Motif XF. This includes all 16 Parts and the A/D Input. All sixteen of the XF PARTs each has its own individual volume control when in Mixing mode. The A/D Input has its own volume control in all modes including Voice mode and Performance mode, as does the selected Voice and each of the four Parts of a Performance.

However there is only one main Volume slider through which all Parts (including the A/D Input) travel.

VOICE MODE
In VOICE mode, each Voice can be made up of eight Elements. Each Element has its own individual output level control (via the Control Sliders 1-8) and each Voice has its own output level parameter that affects all eight Elements together:
Press [EDIT]
Press [COMMON EDIT]
Press [F1] GENERAL
Press [SF2] PLAY MODE
Here you will find the OUTPUT LEVEL that controls the internal Voice's volume output level. The master volume of the combined 8 Elements together.

To control the A/D Input Parts volume throughout VOICE mode
Press [UTILITY]
Press [F4] VOICE AUDIO
Press [SF1] OUTPUT
Here you will find the Volume for both the analog (A/D) and digital (FW) Parts.

PERFORMANCE MODE
In Performance mode each Performance can be made up of four Parts. Each Part has its own individual output level control (via the Control Sliders 1-4) and each Part has its own output level parameter that affects all four Parts together:
Press [EDIT]
Press [COMMON EDIT]
Press [F2] LEVEL/MEF
Here you will find the Volume Output Level that controls affects all four Performance Parts together

To control your external audio inputs (this is handled on a "per Performance" basis:
While in EDIT > COMMON EDIT
Press [F4] AUDIO IN
Here you find the AD and FW Part's Volume Output Levels... These are storeable per Performance.

And finally all Parts, both internal and external, travel through the main Volume slider.