Unfortunately, that functionality hasn't been added to a PowerShell cmdlet, which means it isn't easily doable in DSC. You'd need to code up the necessary command, and then create your own resource around it.
In general, when you need certain action done that isn't found natively in the DSC resources, try finding a way to do that action in normal powershell and then try using the built-in Script DSC resource for this.
Based on my search, this is something you can do:
[System.Reflection.Assembly]::LoadWithPartialName("Microsoft.Web.Management") [Microsoft.Web.Management.Server.ManagementAuthentication]::CreateUser("MyUser", "ThePassword") [Microsoft.Web.Management.Server.ManagementAuthorization]::Grant("MyUser", "Default Web Site", $FALSE)