Yes, but after the cruise has started the reimbursement would be added to your OBC, so you would then have even more OBC to spend... Only if you cancel before the cruise starts, will the reimbursement go back to your original form of payment. Depending on what kind of OBC you have (refundable/non-refundable, from the cruise line, from your travel agent, as compensation for something, etc.), it may be possible to call NCL and ask for the shore excursions to be paid with your OBC.