First, IMAP does not provide access to server contacts.
There are other, unrelated, network protocols for that -- if you're coming from GMX's or Fastmail's app, then those have it built-in of course, but for us peons third party developers, things are different.
Second, you can easily save the sender's email when viewing an incoming message. Just tap the sender's image (or placeholder / color chip).
Third, Aqua has a setting to automatically save the recipients of your messages -- as you send them -- in system contacts, under the first @gmail account (where they will be synced to Google's servers). This is off by default, to enable, app settings -> contacts -> "auto-add contacts".