Assuming you have a list of usernames, each on its own row in userlist.txt, the process is not too complicated.
# Array for name&sid tuples
$users = @()
cat C:\temp\userlist.txt | % {
# Syntax sugar
$spSid = [Security.Principal.SecurityIdentifier]
# Custom object for name, sid tuple
$user = new-object psobject
$user | Add-Member -MemberType noteproperty -name Name -value $null
$user | Add-Member -MemberType noteproperty -name Sid -value $null
$user.Name = $_
$user.Sid = (New-Object Security.Principal.NTAccount($_)).Translate($spSid).Value
# Add user data tuple to array
$users += $user
}
# Export array contents into a CSV file
$users | Select-Object -Property Name,Sid | Export-Csv -NoTypeInformation C:\temp\sidlist.txt