System will allow to add a custom fields in object definition but will throw an error while accessing Income Tax Declaration portlet in employee file or the custom field will not be visible.
Employee Central: Income Tax Declaration
Currently custom fields are not supported for ItDeclaration object. The engineering team is aware of this and there is an enhancement request (ECT-88072) for the same. We don't yet have fix version available for it. Once it is implemented, it'll be included in the respective release notes. You may contact your CSM or call SAP hotline number to know status of the enhancement request.
ECT-88072, error , KBA , LOD-SF-EC-LOC , Localization & Country Specifics , Product Enhancement