Praise be to Allah.
The issue of whether one can add the condition of saying “in sha Allaah” when speaking about matters of faith is one concerning which there has been lengthy and detailed debate among the scholars. The correct attitude in this case is to ask what is meant by the word “mu’min” (believer). If the speaker is referring to the essence of faith, i.e., Islam, without which one would be a kaafir, then in this case it is not correct to say “in sha Allaah.” This would imply some doubt about Islam, which is not permitted; the Muslim should be definite in his faith, and should say “I am a mu’min,” without adding “in sha Allaah.” However, if what is meant is the perfection of faith and the high rank which is above the level of just being a Muslim, then one should say “in sha Allaah,” because otherwise one would be praising oneself, which is forbidden as Allaah says (interpretation of the meaning): “Therefore ascribe not purity to yourselves” [al-Najm 53:32]
If a person wants to say something straightforward to express his belief, then he should say “I am a Muslim.” In this case there is no need to add “in sha Allaah.” And Allaah knows best.