The @ Import directive cannot have more than one namespace attribute. To import multiple namespaces, use multiple @ Import directives.
A set of namespaces can be automatically imported into .aspx pages. The imported namespaces are defined in the machine-level Web.config file, within the <namespaces> section of the <pages> element. The following namespaces are automatically imported into all pages:
System
System.Collections
System.Collections.Specialized
System.Configuration
System.Text
System.Text.RegularExpressions
System.Web
System.Web.Caching
System.Web.Profile
System.Web.Security
System.Web.SessionState
System.Web.UI
System.Web.UI.HtmlControls
System.Web.UI.WebControls
System.Web.UI.WebControls.WebParts