Simple solution would be that I add another element to your original array:
In controller
$scope.companies = $scope.subcontractors + [{name: 'New company...'}]
in html
<select ng-model="selected_company.current" ng-options="company.name for company in companies">
<option value="">Please select a company...</option>
</select>
Or even better would be you only modify the html to the following
<select ng-model="selected_company.current" ng-options="company.name for company in subcontractors + [{name: 'New company...'}] ">
<option value="">Please select a company...</option>
</select>