It may depend on the timing - yes, projects have added credits when providing responses to the clarification requests that came after the initial design phase or initial construction phase submission. At that point you should still have the access to add the additional credits in LEEDOnline.

If you are at the point of assembling the final submission for the construction phase you won't have a chance to receive any requests for clarification from the reviewer, so you need to provide all possible information and supporting documentation that's required.

If you've already gotten all the requests for clarification, submitted your final construction phase credits, and you are at the point of accepting the final review you may need to file an appeal to pursue any additional credits. I haven't heard of projects adding credits at this point other than by filing an appeal, but you'll want to confirm with GBCI if that's your situation.