Like title says , I created a mercenary with IMP profile and seems my answers on profile were so good I was full of goodies , even an hospital, and no place for game to spawn a letter, so now I have done first combat , and remembered the letter after seeing Fatima , I suppose that savegame is screwed since I got no letter ? oh yeah on Wildfire

As I recall from the code, if the IMP doesn't have enough space on his/her inventory, it should start checking the other mercs for free space. So check your other mercs.
If you went in with only your IMP and he doesn't have any space left, I'm not sure what happens to the letter. Can take a look at the code, but JA2:WF hasn't had its source code released, so it is possible that there are a few differences.

well all went good after I used Jape to just add one item : the letter, and after that did not try to thinker anymore with it since it is intended for JA2 vanilla, but for the letter it worked
Now I am playing alruco revisited mod for 1.13 after completing wildfire