If a candidate holds a certificate for sail and wants to revalidate and add power endorsement, the school must make this request.    Only schools may request an additional endorsement for candidates as this is verification that the candidate is indeed qualified to hold the certificate.  Please contact the school where you trained and ask them to certify you for power vessels at the level you wish to add.  The school can then order a new certificate which includes power & sail. 

For example, If you hold a "International Bareboat Skipper Sail" certificate and wish to revalidate and add power endorsement.
The school can verify the power training and place order for new "International Bareboat Skipper Power & Sail".