Using the Excel.Application Com-object converting a csv file to an xlsx file is quite simple, it can be written up in a single one-liner:

((New-Object -ComObject "Excel.Application").workbooks.open('c:\list.csv')).SaveAs('c:\list.xlsx',51)

Now this does leave an Excel process open so if you would like the process to be closed afterwards we could amend the code as follows:

$CurrentExcel = Get-Process Excel | Select-Object -ExpandProperty ID
((New-Object -ComObject "Excel.Application").workbooks.open('c:\list.csv')).SaveAs('c:\list.xlsx',51)
Start-Sleep 1
Get-Process Excel | Where-Object {$CurrentExcel -notcontains $_.ID} | Stop-Process -Force

By first gathering the list of all open Excel processes we can make sure that any previously opened Excel processes are not accidentally closed. That leaves us with a quick and easy piece of code to convert comma separated files to xlsx files on the fly.

For more information about the Excel.Application object have a look at its MSDN entry:
Application Object Excel