Adicione a senha como um NoteProperty no seu loop foreach, usando Add-Member e depois selecione sobre isso.
Então, algo assim:
$pool = Get-ADUser -Filter * -SearchScope Subtree -SearchBase "OU=Test,OU=Users,OU=funny,DC=Corp,DC=funny,DC=com"
foreach ($user in $pool )
{
#MY NAME AND MAIL FUNCTION
$a = Get-ADUser $user -Properties name, mail
#MY PASSWORD FUNCTION
$password = {blah,blah,blah}
$user | Add-Member -Force Password $password
$password
}
$pool | Select-Object name,mail,Password | Export-Csv -path test.csv