I've been using POP to download Gmail to Mail.app since I made the account in 2009. Now, Google required me to use 2-step authentication, meaning I also can only use IMAP for my Google account on my Mac as the POP account configuration no longer can log in.
My emails on Gmail and in the new IMAP account only go back to 2011, so if I only use that account, I will be missing my oldest emails. If I can take the POP mailbox as a starting point, it also won't have to waste space on my Mac with so many duplicate emails. So is there a way to use the old POP mailbox as a starting point, and Mail would know not to re-download those existing emails back into that mailbox, and I would have one single account in Mail.app now on IMAP which has my entire email history?