You could provide users the option to give up their encryption on the content they wish to export. Chances are, if someone is trying to export their entire account, they are moving away from PM. In that case, encryption is probably not their first concern.

Furthermore, even though PM keeps user emails in encrypted form, virtually all of those emails are unencrypted on one or more servers related to the recipients of those emails. So unfortunately, the encryption is less valuable than we would like.