Web resources to append; modify; remove:

http://blogs.microsoft.co.il/blogs/scriptfanatic/archive/2010/02/04/modifying-multivalued-active-directory-attributes.aspx

http://www.powergui.org/thread.jspa?threadID=11973

How to query a multivalued AD attributes with get-qadobject:

(get-qadobject -searchroot ‘cn=users,cn=ncecvsama8,cn=test-gad,ou=zones,ou=unix,dc=nce,dc=amadeus,dc=net’ -includeallproperties) | select-object name,@{Name=’smtpmail’;Expression={[string]::join(“;”,($_.proxyaddresses))}},

However get-qadobject doesnt not work well with all multivalued AD attributes, so as a workaround; use ADSI Moniker see below:

http://jdhitsolutions.com/blog/2011/12/updating-multi-valued-active-directory-properties-part-1/