Read them from where? From the list shown by the Contacts app? Does
K-9 actually have its own internally managed contacts list? If K9 can
read the Contacts app's list, especially if that is all it uses for its
contacts list, then use something that updates that list. You don't
need duplicated contacts within some internal K9 list if can or only
uses the list from the Contacts app.
Since the Gmail app comes bundled with Android, you could use a Gmail
account. You likely already have a Google account since the Android is
pretty useless without one. Go online (web) and use their webmail
client to import contacts into your Gmail account. The Gmail app on the
smartphone will sync to your Gmail account. If you configured the Gmail
app to allow Internet access only via wi-fi (Settings -> Data usage ->
Gmail app, scroll down to "Restrict background data") then the Gmail app
won't sync if the phone only has a cellular connection (and obviously no
sync if no wi-fi and no cellular connection). The contacts up in your
Gmail account will appear in the Contacts app on the smartphone.
What's your reason for not using the Gmail app already bundled in the
Android OS? I haven't bothered looking at using "Inbox by Gmail" to
replace the standard Gmail app in Android. Alas, it only supports Gmail
accounts.
http://www.pcworld.com/article/2935949/5-reasons-to-switch-to-inbox-by-gmail-and-4-reasons-not-to.html
While I have one, it is not used for e-mail other than for voicemail
alerts from my Google Voice account. I use Microsoft's (Hotmail,
Outlook.com) and my ISP's e-mail services. For those, and to utilize
Microsoft's proprietary ActiveSync (crippled Exchange), I use the MS
Outlook app on my Android phone. However, I do import my contacts using
the Gmail webmail client to keep them up to date in that account so they
get synchronized to my phone to show under the Contacts app.