You'll need to do this from a computer.
Go to your Gmail account in a web browser. Choose the gear icon at the top right and go to settings. In settings choose the Accounts and Import, then use the Import Mail and Contacts option. That's as far as I looked, so you'll have to wing it from there.
It might be possible to do this from Safari on the iPad. I did not check that.
The other option is to go to Yahoo Mail in a computer's web browser, select all your contacts, then use the Export option from the Actions menu. Use the single file VCard option and mail it to yourself. On the iPad in the Mail app it should be possible to tap and hold this attachment and get an import option for your contacts. Make sure that your Google account is set as the default Contacts account in Settings > Mail, Contacts, Calendars first (if you have more than one account set up on the iPad).
Or you can go to the Google site and try importing the file there.