A 28-year-old woman from Mumbai with a history of MDR-TB (treated with levofloxacin, amikacin, and ethionamide for 6 months) presents with persistent productive cough and fever. Repeat sputum smear microscopy remains positive. Drug susceptibility testing reveals resistance to isoniazid, rifampicin, fluoroquinolones, and injectable agents (amikacin). She is now classified as having XDR-TB. What is the preferred drug of choice to add to her regimen?