I think that this outcome can be added to the results as post-hoc analysis, but relevant descriptions must be made to clarify that this outcome is a post-hoc one, as well as the reason why you did not include it in the first place, and why you would add this outcome now.

Thanks, Qiuhe Wang.... i was thinking, peers will reject my work, but as it was a important outcome that i observed thus could not resist to include that in publication.

What happened to the outcome that you have stated in the registered trial? Are you including it in the publication? if you are, I do not think peers would reject your work as long as you point out (as Qiuhe has pointed out) that you have determined to analyze the new outcome after looking at the data. Evidence from such analysis is of course weaker and is only exploratory (not confirmatory) because you can not generate a hypothesis and confirm it from the same data set, you may be committing Texas sharp shooter bias. That is why it is very important to point that out. It would be worse if you are not reporting the outcome stated in the registry or if the results contradict each other.

@Muhammad M hammami, thank you sir for information. I am including outcomes registered in clinical trial registry.

I think reviewers will understand this as long as you provide sufficient reasoning. Such things do happen in clinical trials, since people's recognition may change as time goes by, and clinical trials usually takes a lot time. Moreover, this new outcome you would like to add is a post-hoc secondary outcome, it doesn't influence your primary endpoint and main conclusion -- as Muhammad mentioned, it is just an exploratory finding.