You can use FTP to upload a skin in html format
If you FTP a uniquely named folder underneath ...portals/_default/Skins, DNN will recognise this folder but it will not detect any skins (because none have been created yet).
If your FTP'd folder includes a skin.html, simply reparse the skin from Host>Skins, this will generate the skin.ascx
The reparse text link will show once you find the folder name from the skin dropdown list at the skin management page.
If you FTP to a portal specific then you would do the same thing but upload the folder to underneath ...portals/X/Skins and parse it via Admin>Skins.
Regards,
Steven