$property_list = ($csv_import | get-member | where-object { $_.MemberType -eq "NoteProperty" }).name

# Create the property for the select-object
$expression_list = $property_list | foreach {
    if ($_ -eq "email") {
        @{
            Name       = ('C_{0}' -f $_)
            expression = [Scriptblock]::Create('$_.{0}' -f $_)
        }
    } else {
        @{
            Name       = ('{0}' -f $_)
            expression = [Scriptblock]::Create('$_.{0}' -f $_)
        }
    }
}

# You dont need to join here, it's allready an array
#$expression_join = $expression_list -join ","

# working as expected
$csv_import | Select-object $expression_list