Can I attach previously sent emails to a new email in Gmail?
oops! simpler one is there :
go to sent mail, click on more click on forward all .
this should work to forward the thread, whehter it is sent mail or inbox.
what do you exactly want to do ?
just forward or reply to sent mail...and add your text.
if you want to attach the sent item and send it to the same person to which the sent mail is sent, then you just have to open the sent mail and click on reply
gmail automatically sends the conversation,
you can try the same option when sending the sent mail.
or simply copy and paste the content.