How do you add an email to a project or opportunity.

Quick Base supports native Sync to Gmail and Outlook.  If you can arrange to have emails forwarded to say a gmail account for this purpose and can ensure that say the Subject field includes the Project# in a standard format like Project# 1234, then you can have a Report Link field on the Project record display relevant emails. 

It will not be a true relationship, but you do not need to have a true relationship to have a Report Link field and the matching does not have to be exact for a report link field to work.  There is a a setting for that on the report link field.  ie the subject can read "Cost overruns on Project# 1234" and that is good enough to get a fuzzy match against Project# 1234