Flaper, I will say you can do it (unofficially): this can be achieved by the multipad audio link.
1. Press the select button by the multipad (this feature will
be unavailable in styles recording mode etc.).
2. You be able to see the option to press the audio link mode for the multipad.
3. Select the audio file you would like to have recorded with performance (in your case
sing-along) in one of the empty four slots.
4. Once assigned, you can start recording audio and simultaneously let play the audio assigned to one of the multipad buttons!
The cons you will figure it out yourself… But it does the job for you and better than nothing
mendel