Put a sensible guess as to the steps/mm. Almost anything will do, as long as material comes out of the nozzle at a reasonable rate when you command the extruder to move.
Test how many "commanded mm" it takes to output say 100mm of material.
Refine your steps/mm according to the standard extruder calibration methods, e.g.
New steps/mm = Old steps/mm x 100 / measured material extruded
Repeat.There is a fair amount of guesswork, as you need to take things like stretching of the filament due to gravity and die swell into account but it shouldn't take too long to refine a useful number. If you then set your "filament" diameter to your nozzle diameter in the slicer then everything should work out, shouldn't it?