Out of the four available finishers for that model, the FS-503 Staple Finisher (100 Sheet Stapler) is the only one that doesn't support half-folding.
SD-501 Saddle Stitcher: Half-folds and center-staples up to 50 sheets for 200-page booklet-making ─ with nested half-fold and letter fold, trimming and automatic image shift for perfect gutter alignment.
FD-501 Multi-Folding Unit: Provides 6 folding types, including half fold, gate fold, letter fold-in, letter fold-out, double parallel fold and z-fold plus post-process document insertion and 2/3 hole-punch capability.
FS-607 Staple Finisher: Small-footprint finisher that staples up to 50 sheets, lets you create saddle-stitched booklets of up to 80 pages (20 sheets), and offers tri-fold and half-fold capability.