Hi, is there a way we can script to add a "department" on one line on GL Impact?

Yes, you can do this with SuiteGL, using the Custom GL Lines Plug-in. page 41 - setDepartmentId(departmentId) https://docs.oracle.com/cloud/latest/netsuitecs_gs/NSCGL/NSCGL.pdf

@NetSweetie thanks. Can we do this using suitescript api? or SuiteGL is the only way?

SuiteGL is a real and available thing. In NS, navigate to customization > plug-ins > plug-in implementations to view/create SuiteGL scripts