Omissions in papers filed by the applicant in conjunction with an international patent application (e.g., correspondence), other than the international application itself, may be remedied. The letter remedying the omission must be filed within the time limit fixed in the invitation to do so. The time limit is typically 1 month.
The position of the USPTO on this issue is described in the following section(s) of the Manual of Patent Examining Procedure (MPEP):
MPEP 1834 - Correspondence
Law - PCT Invitation To Remedy Omission