No. After that episode the sole mention of Unimatrix Zero is in the alternate timeline seen at the start of the episode "Voy: Endgame". There's a fleeting mention of the Borg Resistance but no mention of Axum.
Yes. In the novel "Star Trek Voyager: Acts of Contrition", Seven-of-Nine (AKA Annika Hansen) meets up with Axum at a Starfleet medical facility.
Seven turned from the view she had been studying: a large, furnished patio teeming with botanical life accessible to several other adjoining quarters.
Standing before her, at long last in the flesh, was a man she had only ever known in dreams.
“Axum,” she said softly.
Seven did not bother taking his hands in hers. She simply opened her arms as he half fell into her. Her strength held both of them upright as his gentle embrace became more intent. It might have been hours that they stood like that before they finally parted to look again into each other’s eyes.
“Where are we?” Seven asked, to break the tension.
Axum stood on his own feet, but he still held Seven by both arms. “I live here now,” he replied.
Offering any further info would instantly spoil the book, but if you want to read a more detailed review, have a glance at the novel summary