As I know, the EXIT_SAPLMV01_003 is only to modify additional data segments isn't it?
No, You can change the standard segment also. since you were able to trigger the idoc, keep a breakpoint in the exit and in debugger mode try to change the value of segment(the one which you want to change). If it allows you to change that means you can write your code in that exit.
OR
If the field value which you want to change is available in BAdI ARTMAS_DATA_ENRICH, then implement this BAdI.