iOS whatsapp offline pending messages not supported
Here in iOS that's not currently possible, in fact when you've no connection and try to send a message, the Send button will be disabled, and even though you could successfully tap it before the connection goes off, the message wouldn't get sent and a red icon would appear on its right side, to alert you it was not sent and it won't until you don't get connection back.



Summary

So the only messages which will be sent when you get the connection back will be those who were sent when you had actually a connection, but got lost during the process of sending the message.

Instead you won't be able to do that when you have no connection at all.



Though, developers love to hear their customers feedback, so I highly suggest you to give it a try and contact WhatsApp support (support@whatsapp.com) for iOS and let them know your idea, who knows it may be added in the next update hopefully!

Hope this helped you out!