[IMP 5.x] Fetchmail/Getmail like functionality has been removed from IMP 5 Ticket #8077.
[IMP 4.x] IMP can "Fetch mail" from other accounts and merge it into your INBOX or other folder(s) displayed by IMP. If you want to manage them separately rather than merging them all into one folder, you can simply create a separate folder for each account, and then configure IMP's Fetch Mail to download messages for each account to its own unique folder. Each folder would then represent an account. Using multiple identities tied to each account's email address, you can set it up so that when you reply to a message from any given account, the proper From: address is used.