When the import screen is displayed, look at the top of the right hand side for the File Handling Panel, and select "1:1 previews" there. Lightroom should then build the previews for all of the imported images once the import completes.
Making across the board 1:1 Library previews seem to me an incomprehensible waste of time and disk space. Will you really zoom to 1:1 in the library loupe view for thousands of shots? Why else do you need high rez previews? Better to wait a few seconds for one to be rendered if and when needed.