Hello Lee,
thank's for the tipp, I did it :-)
It was finally very simple, I override the css tag definitions used for the edit mode within the little script in my skin and now it's working.
<% If DotNetNuke.Security.PortalSecurity.IsInRoles(PortalSettings.AdministratorRoleName) %>
<% End If %>
Just I had to set overflow-x within my wrapper, otherwise the horizontal scrollbar is not shown, but I think that's only a little problem what can wait to be solved.
Best regards
Daniel