Actually, you can add LORs after eSubmitting. You just can't change anything after the LOR has been completed by the person writing it. If they haven't filled it in yet, you can delete them and replace them with someone else any time you want. They process the app whether all of your LORs are in or not and will just include any incomplete ones as they are completed.
A friend of mine had to do this. The email address of one of her LOR writers wasn't able to receive the OptomCAS request. She called OptomCAS to make sure she would be able add the LOR request again if she deleted the current one and re-added it with an alternate email address. They assured her that the system would allow her to Add an LOR even though she had already eSubmitted and she was able to do it. As mentioned though any LORs that are complete are locked and can't be changed or deleted. But if it's not completed, you can delete and re-add if necessary and you should also be able to add additional LORs if you have any spots left (their system takes a maximum of 4 LORs). If you have doubts just call them to make sure.