It would be "OK" if and only if you obtain written permission (in email) from the original author of the skin, and include it in the zipped file. It would also be good if you were, after obtaining that permission to summarize and make clear the changes you made to the skin in your written comments, and provide a link to the original skin. Also, it would be good form to send your modified skin to the original author to see if he/she approves of the changes you have made.
If you cannot obtain the permission, you may not submit it for publication.
You can find more here: https://www.wincustomize.com/upload-guidelines
edit: *dang these old, slow fingers. doc waves at Bry.
