Can a PowerShell DSC configuration file be created from a current system build?

Not directly. You'd have to approach each resource you want to module independently.

For example, if you want to model the existing windows roles and features, you could script out something like

Get-WindowsFeature -ComputerName ny-web01 | 
? installed |
% {$t = ''} { $t += @"

WindowsFeature "Role-$($_.Name)"
{
    Name = '$($_.Name)'
    Ensure = 'Present'
"@ 
    if ($_.dependson)
    {
        $t += @"
    DependsOn = '[WindowsFeature]Role-$($_.Name)'
"@
    }

    $t += @'

}
'@
} {$t}

Each resource will be unique in how you want to identify those things you want to control.