You can definitely connect your Apple Mail account to Nimble via IMAP. Take the following steps to connect:
In Apple Mail, go to Preferences >> Accounts.
Copy down your email account information and translate it over to Nimble.
(Please note when Apple's two-step verification is on, it may block Nimble from accessing your account).
Once your have your IMAP credentials, log into Nimble and visit the Settings >> Networks & Imports page. From here, you'll be able to connect the account via the IMAP connector. Learn more about connecting to Nimble via IMAP here.