You can only do this in PHP if all of the sub fields will have the same default value. You really don’t need to add anything if all of the sub fields will have the same value.
You can only generate sub fields with different values in PHP if you create enough rows in the repeater to hold those values.
If you want to generate different default values for each row of sub fields then the only way to do that would be to do that using JavaScript/jQuery/Ajax.
There is not much in the way of examples for doing the last.
