There could be many solutions, if the requirement is ONLY to maintain an Additional Personnel / Reference Number, such as
- IT 0185 where you could create additional Personal IDs.
- IT 0032 where you could store another Personnel Number.
By storing additional info in this manner, you could have one payroll result.
However, if the employee has different employments within the organization, you could utilize SAP's Concurrent Employment Solution.
Reference Personnel Number (31) would be used if an employee who left the organization earlier joins back.