I think the answer must be No, kind of.
If you modify the incoming attachment, logically it isn't the original any more - as Zelandakh points out, attachments are normally opened in the temp folder, so any modifications you make will be reflected in that version. Even if you save the changes, the 'real' original will be unchanged.
You could cause yourself problems - if you modify/save the opened attachment in the temp folder, then re-open the mail and the attachemnt, you might trash the modified version. I think it's probably safer to save the attachment somewhere else, modify it then attach it to your reply - As suggested by Zelandakh.