An important information is missing from my paper under review I have a paper under review (Second round) after a minor revision. The journal has limited page length and I contacted the editor to ask whether I can add additional pages but he said that it is only possible after acceptance. One information which I thought less important seems very crucial, where other researchers asked me about it after a presentation of my research. In addition, the lack of this information in the implementation will completely deviate the experimental results. 

I am wondering how I can add it after the acceptance of the paper. Should I inform the editor for another review round?
Since it is a crucial piece of information that alters the content (and possibly the result) of the paper, the most fair would be to contact the editor and explain the situation so that the reviewers (which seem that did not catch the missing piece) make a better and more informative decision. 

The new version should be refereed. I think just adding supplementary material on an accepted (but possibly flawed or at least weaker) version is not suggested.

So: Contact the editor. S/he probably let you submit a new version which will go to the usual review channels, probably with the same reviewers. As for the page length, probably you will have to squeeze in everything, which means a lot of re-writing.