first naive attempt to have a offset in the homeing procedure. Does not enable to move into regions not allowed by endstops.

Marlin/Marlin.h

 extern float homing_feedrate[];
 extern bool axis_relative_modes[];
 extern float current_position[NUM_AXIS] ;
+extern float add_homeing[3];
 
 #endif

Marlin/Marlin.pde

 // M203 - Set maximum feedrate that your machine can sustain (M203 X200 Y200 Z300 E10000) in mm/sec
 // M204 - Set default acceleration: S normal moves T filament only moves (M204 S3000 T7000) im mm/sec^2  also sets minimum segment time in ms (B20000) to prevent buffer underruns and M20 minimum feedrate
 // M205 -  advanced settings:  minimum travel speed S=while printing T=travel only,  B=minimum segment time X= maximum xy jerk, Z=maximum Z jerk
+// M206 - set additional homeing offset
 // M220 - set speed factor override percentage S:factor in percent
 // M301 - Set PID parameters P I and D
 // M400 - Finish all moves
 int saved_feedmultiply;
 volatile bool feedmultiplychanged=false;
 float current_position[NUM_AXIS] = {  0.0, 0.0, 0.0, 0.0};
-
+float add_homeing[3]={0,0,0};
 
 //===========================================================================
 //=============================private variables=============================
       }
       feedrate = 0.0;
       home_all_axis = !((code_seen(axis_codes[0])) || (code_seen(axis_codes[1])) || (code_seen(axis_codes[2])));
-
+      
       if((home_all_axis) || (code_seen(axis_codes[X_AXIS]))) 
       {
         HOMEAXIS(X);
+	current_position[0]=code_value()+add_homeing[0];
       }
 
       if((home_all_axis) || (code_seen(axis_codes[Y_AXIS]))) {
        HOMEAXIS(Y);
+       current_position[1]=code_value()+add_homeing[1];
       }
 
       if((home_all_axis) || (code_seen(axis_codes[Z_AXIS]))) {
         HOMEAXIS(Z);
+	current_position[2]=code_value()+add_homeing[2];
       }       
+      
       feedrate = saved_feedrate;
       feedmultiply = saved_feedmultiply;
       previous_millis_cmd = millis();
         st_synchronize();
       for(int8_t i=0; i < NUM_AXIS; i++) {
         if(code_seen(axis_codes[i])) { 
-           current_position[i] = code_value();  
+           current_position[i] = code_value()+add_homeing[i];  
            if(i == E_AXIS) {
              plan_set_e_position(current_position[E_AXIS]);
            }
       if(code_seen('Z')) max_z_jerk = code_value() ;
     }
     break;
+    case 206: // M206 additional homeing offset
+      for(int8_t i=0; i < 3; i++) 
+      {
+        if(code_seen(axis_codes[i])) add_homeing[i] = code_value();
+      }
+      break;
     case 220: // M220 S<factor in percent>- set speed factor override percentage
     {
       if(code_seen('S'))