Add-member for Powershell V2

Thanks to this forum, I have a script to sort an array of fully qualified file paths by just the filename portion.

            $pattern = [regex]".+\\(.+)$"
            $files = $files | Add-Member @{BaseName=$null} -PassThru
            for ($i = 0; $i -lt $files.Count; $i++)
            {
                $files[$i] -match $pattern | Out-Null
                $files[$i].basename = $Matches[1]
            }
            $files | sort BaseName

However the Add-Member fails when run on a machine running Powershell V2 - error below - can someone help me to reformat this line to run successfully on a V2 box?

Very new to Powershell...and very grateful for all your help!

Cannot bind parameter 'MemberType'. Cannot convert the "System.Collections.Hashtable" value of type "System.Collections.Hashtable" to type "System.Management.Automation.PSMemberTypes". ()