If you want them all to be IMAP based, you're probably going to have to set up some forwards for email coming into accounts like Comcast, where they probably do not offer IMAP support (maybe if you pay for it).
Gmail offers it for free (and just FYI Google has reserved the right to charge for it at any time) and you can enable it in the Gmail settings. So you could potentially forward all your other messages to your Gmail account, then set up filters to dump mail from say your Comcast account into a Comcast label/folder.
The big problem with this, is that if you respond to any email, it will have your Gmail address as the return address, and there's really very little you can do about that. Using a local email client you can set the reply-to field, but you'll have to set that to the respective email account manually.
No matter how you go about this, it's going to be messy unless you don't care if people send you mail on your Comcast account and get a response back on your Gmail account.