This functionality is not supported out of the box for the Xamarin.Forms DataForm. I have logged it as a feature request in our internal system. You can track its progress on the following link from our Ideas & Feedback
portal - DataForm: Add support for enumerable properties
. I have added some points to your account for bringing up the issue.
In the meantime, you can create custom renderers for the different platforms and return a custom editor for the specific property. There is a sample on how to do this in Android in the "Samples" application which can be found in the Examples/Android folder of your local Telerik UI for Xamarin installation. You can have a look at the following article for custom editors in iOS - Email and Password editors in iOS
Do you want to have your say when we set our development plans? Do you want to know when a feature you care about is added or when a bug fixed? Explore the Telerik Feedback Portal
and vote to affect the priority of the items