Theoretically? (If no, why? And if yes, why hasn't it been done?)
Could IMAP authentication be adapted to support 2FA?
IMAP can support multiple authentication methods, i.e. the protocol is extensible.
... why hasn't it been done
It has been done. Modern clients like Thunderbird support XOAUTH2 which basically uses a web client with OAuth2 which then can do all the normal things OAuth2 can do including 2FA.