Any ideas how to do that guys?
Assuming you use a Mac. Go to the Contacts app on your Mac and export your contacts to a CSV file. Log on your Gmail and in the Contacts section, click the option for import. A window will pop up asking for your CSV file. Click that to upload it to Google and you'll have your contacts imported. Over on your Nexus 7, the contacts should sync over shortly.
You can also try transferring the contacts from your iPhone to Nexus 5 easily, with some transferring tool like Phonetrans for iOS. This tool will allow you transfer data such as videos, music, photos, contacts, text messages, etc from iPhone devices to Android devices easily.
Are you sure? There isn't any option to export to CSV.
Ooops I meant as a vCard. Select all you contacts and then chose export as vCard.
Indeed, the export should be in vCard format. For one thing, CSV files do not keep the contact pics.