Converting CSV from Data Table to Key Value Pairs in Powershell

I think you misunderstand how Import-Csv works. It does not create a hashtable, it creates an array of objects. Each object will have a set of properties, as defined by the header. Since the data was imported from a CSV it simplifies things by guaranteeing that each object has the same properties (they may not have values, but the properties exist and are identical). Because of that we can get a list of the properties of the first object as a baseline set, then loop through each record and build a string for each record based on that baseline. As you suggested we would be building the CSV manually.

$DataIn = Import-Csv C:\Path\To\File.csv
$Props = $DataIn[0].psobject.properties.name
$DataOut = ('record,'+$((2..$Props.Count|%{'field,value'}) -join ',')),$(For($i=0;$i -lt $DataIn.count;$i++){
    [array]$tmpRecord = Switch($Props){
        'Record' {$DataIn[$i].record;continue}
        {[string]::IsNullOrEmpty($DataIn[$i].$_)} {continue}
        default {'{0},{1}' -f $_, $DataIn[$i].$_}
    }
    If(($tmpDiff = $Props.count - $tmpRecord.count) -gt 0){$tmpRecord += ','*($tmpDiff*2-1)}
    $tmpRecord -join ','
})
$DataOut | Set-Content C:\Path\To\Output.csv

So that does exactly what I suggested, while retaining your example output of not doing key/value for the record column. The switch checks against each potential property, and if it is the 'record' property it just outputs the record value and continues on to the next property. If it is is anything else it checks to see if that property is blank, and if so it moves to the next property. If it isn't blank it outputs field,value, and then all of those outputs (record, and any field/value combo) are joined by commas into a single line per record. It also adds on extra commas for fields that are null. Each record's line is collected in $DataOut, along with a calculated header line.

Mind you, PowerShell will not want to read that file in with Import-Csv because of the duplicate columns since the header row is mainly 'field,value' repeated over and over. I assume that you are saving in this format for some external program that needs that format for input.