Supporting external imap account is something we have in mind, but it's also going to bring possible out of control issues, depending on the imap implementation of the extenal ISP.
We cannot guarantee performance nor full features, because at the moment we grant NethServer Dovecot and XStreamOS Cyrus Imap installations, both specifically built and configured to work as we expect.
Actually, you already can set up an external imap account on a WebTop account, bypassing completely the internal imap server.
Just impersonate the user via admin (login as email@example.com and using admin password), then go to the settings, mail tab, change all account data into the external ISP data, save, exit, login back using webtop credential for the user.
You will eventually need to check the special folder names.
Let me know if this answers your question.