Forgot to continue... Now, here is how I could solve it:
This is the content of a textfile named "PMDG 737NGX.mcro" which goes into the modules folder:
[Macros]
1=NGX Fuel 1 off=K112,11
2=NGX Fuel 1 on=K113,11
3=NGX Fuel 2 off=K114,11
4=NGX Fuel 2 on=K115,11
This file is loaded with the following statement in the MacroFiles-section of the FSUIPC.ini:
[MacroFiles]
1=PMDG 737NGX
Thereafter, one can assign the fuel valve commands to two analogue axes. In case of the Y and Z axis of my Saitek quadrant, the assignment in the Axes-section of FSUIPC.ini reads as follows:
0=1Y,256
1=1Y,B,-16384,-9752,M1:1,0
2=1Y,B,256,16383,M1:2,0
3=1Z,256
4=1Z,B,-16384,-9752,M1:3,0
5=1Z,B,256,16383,M1:4,0
Michael