Hi,
Yes you can, you just need to count the current number of instances of the subform (data) and then set the focus to a field in that instance:
_data.addInstance();
var i = _data.count - 1;
xfa.host.setFocus(xfa.resolveNode("data[" + i + "].firstName"));
The underscore before the name of the subform is shorthand for "instanceManager".
Sample here: https://acrobat.com/#d=2pAry7PSy7HdLPidEU2XHQ
Hope that helps,
Niall