Is it possible to add additional or custom Genders in the Gender Dropdown during Personal Data Collection Step or New Hire Data Review Step without Employee Central?
SAP SuccessFactors Onboarding
Unless the you have enabled the feature via Upgrade Center then you may not be able to add Other Gender in the drop-down menu.
You may verify using the KBA - https://launchpad.support.sap.com/#/notes/2093276
KBA , LOD-SF-OBX-EC , Integration EC - MPH, Hire , How To