We are going to be hiring a additional employee later this year. Is there a way I can add the compensation expenses for this employee to my budget without knowing who the person is yet?

Yes, HCP allows you to plan for a To Be Hired  (TBH) employee

From the Monthly Pay Rate tab on the Manage Existing Employees and Job Codes task, right-click and select Manage TBH / Add TBH.  Select the DeptID (defaults to value in the right-clicked row), TBH #, Job Code, Monthly Pay Rate, Start Month and Year, End Month and Year, and Comment (optional). Then select Launch to create the monthly pay rate row for the to be hired employee.

Once the TBH is added you will need to add a TBH Distribution by right-clicking over the new TBH on the Monthly Pay Rate tab and selecting Manage TBH / Add TBH Distributions. 

Once the actual employee is hired, you can then assign that employee to the To Be Hired employee. See To Be Hired Employees for detailed instructions.