Can I Add New Employee in Draft/Pending Approval Pay Run?

Yes, and you can change it many times. But first, you should add new employee's information in People. 

Read more: How to Add New Employees?

Next, go to Payroll > Pay Run > Double click the draft you want to change > Click on Add in the step 2

Pay Run: One-time/Add new
​How to Set Up a Commission or Bonus Pay Item?
​How to Add Different Fixed Pay Items to Employees Included in Same Payroll Policy?