What is the layout of the name you used for the daily CSV files ?
Since that filename contains the name of the resourcepool, we could use the Group-Object to group the CSV files that belong together.
For example, assume the CSV filenames are following this layout: cluster-rp1-date.csv
Then you could do something like this
$folder="C:\powercli\stats"
Get-ChildItem-Path$folder-Filter"*.csv"|
Group-Object-Property {$_.Name.Split('-')[1]} |%{
$monthArray=@()
$_.Group|%{
$monthArray+=Import-Csv-Path"$folder\$name"
}
$rp=$monthArray[0]."Resource Pool"
$monthArray|Export-Csv-Path"$folder\$($rp)-Month.csv"-NoTypeInformation-UseCulture
}