Use the 'continue from this clip' feature. Next, merge the newly generated segment with the existing one using the 'get whole song' feature.

Or just get creative, use the vocal remover to extend the background sound, cut and paste different parts of it using the good old DAW. By using this method, i managed to make several "full" songs :

https://soundcloud.com/sulfonilklorida/prompted-melodies

https://soundcloud.com/sulfonilklorida/tax-report

https://soundcloud.com/sulfonilklorida/dance-of-despair

https://soundcloud.com/sulfonilklorida/rebels-of-the-byte

https://soundcloud.com/sulfonilklorida/break-the-chain