Yes, I think you are still in the right time frame. Since you need to put them in lock down on the 18th, or 3 days before hatch day, and the new egg is only 2 days later, you could technically add it today. The newest egg will hatch two days late, but the older eggs should be ok in the bator till the last one hatches. You will have to move them to an brooder right after the last guy hatches so they dont dehydrate or get hungry. Keep a close eye on them since the older chicks may peck the last hatcher.
The one you put in today could hatch a day early (ya never know) so it would e even closer in age to the others. I would NOT suggest adding any after today as the lockdown and hatch dates will just be tooo far away from each other to be safe.
So, what are you hatching?