You will have to amend your return to add the additional W-2s. Amended returns must be filed on paper, and generally cannot be filed till after the IRS processes your original return.
Please note that due to changes this year in how quickly W-2s and most 1099-MISCs are reported to the IRS by employers (an anti-fraud measure from the same Act of Congress that delayed EITC & ACTC refunds till mid- to late February), it is now far more likely that the IRS will catch your missing W-2s and send you a nasty letter instead of the refund you expected. Even more so than ever, it is *ALWAYS* a good idea to wait for *ALL* your W-2s & 1099s to come in before filing; employers & other payors still have till January 31 to put them in the mail (plus usual USPS delays).