(Merlin firmware)
I'd like to add some gcode that goes "outside" the usually allowed working X axis, - so the head can be cleaned.
- or should I configure Merlin as if X-axis is extra long, longer than working area ?

There is g-code to set the current point to any value. If you move the head to the edge of the bed, set the current position to a point not at the edge of the bed - then you can move the head further. This is a slightly risky technique though!

It would be better to set the limits further in Marlin, and then make sure your slicer is set to not exceed the normally printable area.

Thanks.