Save them to your SIM and then use the Settings>Mail, Contacts and Calendars>Import SIM contacts option
Or if you have synced your contacts with Google or a Microsoft Exchange server
On your iPhone, open Settings > Mail, Contacts, Calendars. Tap Add Account and then select Microsoft Exchange. Enter your email address and login information. (If an Exchange server hosts your email, you'll also need to enter its domain; if you use Gmail, leave that field blank.) Enter a usernamea valid name if youre syncing with a real Exchange server, or your email address (if you like) if you use Gmail. In the Server field, enter the Exchange servers address or m.google.com for Gmail. On the last screen choose what you want to import, and then tap Accept.