XL-Desk has the equivalent of Cue To Mix, as found on the Matrix and X-Desk consoles, although it's not called Cue To Mix on XL-Desk.
XL-Desk's MIX A bus is typically used as the main mix bus. The Cue bus can be sourced from the "alternate" inputs (meaning either the Line input or DAW input of each mono channel strip, whichever is not routed through the main channel strip), and the Cue bus can then be routed to MIX A, providing 16 additional mixdown inputs.
16 mono channel inputs, plus 4 stereo channel input pairs, plus 16 alternate inputs routed via the Cue bus, provide 40 inputs at mixdown.
By also using the stereo returns and insert Sum functions it's possible to squeeze in over 50 summing inputs.