Adding Site Administrators to a Site Collection

If you want to add a SiteAdministrator to a SiteCollection you can't add the user to the SPUserCollection SPWeb.SiteAdministrators.

SPWeb.SiteAdministrators.Add()

SPUserCollection SPWeb.SiteAdministrators is of type SPUserCollectionType.UCT_SiteAdmins (6), which appears that SharePoint is not taking care of.
If it's a bug, i don't know.

Instead you have to set the property IsSiteAdmin of the SPUser to True;

SPWeb web = SPContext.Current.Web;
SPUser user = web.AllUsers["loginName"];
user.IsSiteAdmin = true;
user.Update();