If you think you are short on points, you can always attempt to send them an update to your file and see if they add it on there but don't count on it. It's your duty to make sure you have enough points before you apply. If they started letting people add points after the fact, they would have a stack of incomplete applications where people are waiting for a couple of more years experience or education or better IELTS or whatever.
The above point is only with regards to work experience, not with IELTS etc. (NOTE: b/w application and assessment)
. And it is justified by CIC Manuals as well as on the web-sites of Immigration Lawyers and consultants.
However, This is only favorable to Applicants who at the time of application could not claim 21 points (full points for experience).
Once assessment has started u cannot add anything to it (Work Experience).