Salaried employees charging hours

If you can or if you are allowed to are two different things. A salaried person expects full pay no matter how many or few hours they put in so you better get an agreement in writing to change your hiring agreement with each. That is the tough part. The easy part is go into their payroll setup, delete the salary item and add Hourly as a pay type.

You do realize that certain salaried employees were possibly still subject to Overtime rules unless their salary was greater than $47,476 (used to be around $23k)

You can enter hours for salaried employees, but that won't change their salary, and should not.  If needed, you can change their salary and work expectations (e.g. work only 3 days a week or something like that) with an HR action, and then change the salary in in QB to reflect that.  

If you start paying your salaried employee an hourly wage, then they will not longer be considered salaried employees, and you may be subject to paying them overtime and so on for a look back period, such as a year.  Lots of labor related issues in doing this.