In this document is explained how to adding the additional fields (like custom info type fields) for selecting the Employee number in free search (Object manager) in PA20.
When you click on the free search option system internally calls an ADHOC query. These entries for each scenario (like Employee, Applicant) will be maintained in T77OMAHQ_FUNCAR. If there is no entry in this it calls the default one /SAPQUERY/HR_ADM.
Go to SQ02 and copy /SAPQUERY/HR_ADM to ZHR_ADM, we can add the custom info type fields in the new one.
Add the info types from Edit -> Info type selection
Select the required Info type from further info types
Make and entry in T77OMAHQ_FUNCAR
Now go to PA20 and check we can see the newly added fields.