Prepare
Extend artwork past the trim by mirroring, stretching or repeating the edge. Existing bleed is detected first, so nothing is added twice.
A file with 3 mm of bleed is left alone; one without gets it added.
Mirror the edge for photos, stretch it for flat colour, repeat it for patterns.
The trim box stays put and the bleed box grows, so the imposition tools read it.
The defaults come from your file. Change a setting and the proof updates before you let go of the mouse.