You can now import contacts from YouMail on iOS.

The new YouMail app version has the ability to import your contacts from the YouMail app to your device.

Navigate to Contacts

Enable the Easter Egg Feature

Within the contact list, at the top, tap on the word people five (5) times until a menu at the bottom pops up.
Tap on Import YouMail Contacts and the contacts from the app will download to your iOS Contacts app.

Notes:

Contacts can be edited via the app or the online account. Additionally, if you edit the contact from your phone's contact list and resync your contacts, the updates will be brought forward to YouMail.
Duplicate contacts (contacts with the same phone number) can cause issues with the greeting played, so please make sure to clear any repeated contacts.
Unfortunately, if a contact calls an Extra Line and they have a custom greeting assigned, the custom greeting does not apply (by design).