Have not tested it by myself. 
http://blogs.msdn.com/b/alexj/archive/2009/06/05/tip-23-how-to-fake-enums-in-ef-4.aspx
Create complex type definition in your edmx.
<ComplexType Name="IsActiveWrapper" > 
          <Property Type="string" Name="Value" Nullable="false" /> 
</ComplexType>
Create complex type 
public class IsActiveWrapper
{
    private bool isActive;
    public string Value
    {
        get
        {
            return isActive ? "Y" : "N";
        }
        set
        {
            isActive = "Y".Equals(value);
        }
    }
    public bool IsActive
    {
        get { return isActive; }
        set { isActive = value; }
    }
    public static implicit operator IsActiveWrapper(bool isActive)
    {
        return new IsActiveWrapper { IsActive = isActive };
    }
    public static implicit operator bool(IsActiveWrapper wrap)
    {
        if (wrap == null) return false;
        return wrap.IsActive;
    }
}
Now you can do something like this
public class TestIsActive
{
    public virtual IsActiveWrapper IsActive { get; set; }
}
var test = new TestIsActive { IsActive = true };