One big caution on the "automatic import" of contacts for the People app is this only works if your e-mail client was set up (or capable of) using the contacts associated with your e-mail account and syncing them.
There are a number of e-mail clients that don't work that way, and when I was last setting up Thunderbird it didn't work that way. Just a few days ago I had to tweak a client's Thunderbird setup because the e-mail account she had been using through an ISP no longer exists, and she was terrified that all her contacts were going to go when that account went. They didn't, though, because her contacts (or, perhaps, addresses - I can't remember which terminology Tbird uses) are stored local to Tbird and are not synced with her e-mail accounts. Since I'm the one who installed Tbird for her several years ago and I know I didn't change any defaults related to how contacts/addresses are handled, it appears that they are managed locally.
When that's the case, you need to make certain to export from your e-mail client and import via the web interface to the contacts for the given e-mail account you're going to be adding to the mail app, and, by extension, the People and Calendar apps. Whether you do this prior to adding it or after will be irrelevant because as soon as those contacts are imported into the contacts list maintained by your email server they will sync with the People app.
I hasten to add this is also likely only applicable to IMAP access, though I'm not 100% certain on that.
Never underestimate the difficulty of changing false beliefs by facts.
~ Henry Rosovsky