You have to create a copy of the skin you already have and install that as either as a host or a site skin.
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Skin Administration
DotNetNuke currently support 2 kinds of skin package installation.
Host and Site skin installation.
You can only install skin packages if you are logged in as Host.
Host Skins
These skin packages will be available for all portals on a DotNetNuke installation.
So any portals on the installation can access / use theses skin packages.
File locations
Skins: /Portals/_default/Skins/"SkinName"
Containers: /Portals/_default/Skins/"ContainerName"
Install a Host skin
A. Login as Host
B. Menu: Host > Extensions
C. Click the "Install extension wizard" link (or select this from the Action menu).
D. Browse the skin package.
E. Click "Next"
You will see a warning if you are trying to install a legacy (DNN 4) skin package.
F. Select the "Skin" or "Container" radio button and browse to the package (again).
If you are installing a "Skin & Container combined" package, you should select the Skin radio button.
G: Click "Next" until the skin is installed.
Site skins
These skin packages are only available for the portals they are installed on
Install a portal skin.
File locations
Skins: /Portals/"PortalFolder"/Skins/"SkinName"
Containers: /Portals/"PortalFolder"/Skins/"ContainerName"
Install a Host skin
A. Login as Host
B. Menu: Admin > Extensions
C. Click the "Install extension wizard" link (or select this from the Action menu).
D. Browse the skin package.
E. Click "Next"
You will see a warning if you are trying to install a legacy (DNN4) skin.package.
F. Select the "Skin" or "Container" radio button and browse to the package (again).
If you are installing a "Skin & Container combined" package, you should select the Skin radio button.
G: Click "Next" until the skin is installed.