I might need to hack Importer to make it allow importing members without requiring an email. Is it even possible ?
(related to this question)
Hrmm. I think the first-party member module really depends on having an email address for an account, and really scanning through it to modify it is both a very bad practice and impractical. What I would suggest is generating dummy email addresses for your source file using a GUID for both the localpart and the domain, making it a statistical extreme that their is any collision between your fake addresses and real ones.
However, this does beg the question; How will you update these email addresses when the time comes? These are websites we're talking about, and I'm not sure but how does Importer deal with passwords? Do you have the passwords in plain text? If not, every imported user is probably going to need to use the EE password reset functionality, meaning they'll need a valid email address (or you go through and set temporary passwords you share with them, on a case by case request).
Based on your comments, your major issue is a bad data set. I'm assuming you are tasked with doing a recovery for someone, otherwise you need tell them you need valid data or no-dice. I looked, and Importer has no extension hooks in it, so if you are really stuck having to deal, just write your own import script. It won't be fun, but then you can do as I said; generate GUID@GUID.com (google GUID if you don't know what that is) for each row you import and use that as your email.