I didn't know if there was a way to slot as part of the initial operation. Or if a second op would be necessary.
There's no slotting attachment in my DSMA brochure. But there may be a couple of ways to Rube Goldberg it yourself if your other operations allow the use of a slide.
First off, the DSMA has three independent slides plus the turret ram. The DSM's front/rear cross-slide was separated into independent slides for the DSMA, plus you have the vertical slide which can be used for form tools (ASM-C6, if memory serves) as well as cutoff (ASM-13).
Secondly, it's routine on a machine like this, if you can't fit a tool to directly do the job, to use a slide motion and a ram motion together. One motion moves a swiveling or sliding toolholder into position, and the second motion bumps or pushes the toolholder through its active motion.
On the DSMA, you are not going to shape or shave a slot one chip at a time. It's got to be done in a single pass. So you have to use a rotary tool of some kind. You can make a tool holder for a sturdy Foredom or similar shaft-driven tool with a small slitting saw mounted. Saw needs to be "small" to avoid tool intererence. Because the saw is "small", you can't just plunge it into the screw head because the cut radius is too pronounced, so you have to traverse the saw through the screw head to get a flat-bottomed slot.
For safety, rig up both a definite manual OFF switch that overrides everything and also an automatic ON switch that only activates the tool during the proper machine cycle. (Hint, it was pretty common to mount bars or levers of various shapes to the turret to activate expanding die head openers.)
If your other operations leave a slide free (and if travels and tool interference also permit), you can mount the rotary tool to the slide, program a spindle stop with appropriate dwell, program a slide cycle that passes the slitting saw through the head of the screw. That's the simple case.
If your other operations leave a turret position open (etc), you can mount the rotary tool to the turret in a holder with enough side travel to accomodate the width of the screw head. Mount a bumper on one of the slides. Program a spindle stop with appropriate dwell, advance the ram with appropriate dwell, cycle the slide to push the rotary tool holder through its secondary motion.
Note: The DSMA did have an automatic face relieving tool holder (ASM-C5) which I have never seen. The mechanics of that tool holder could probably be adapted to the screw slotting task. It's mounted to the front slide, but the axial motion (slitting saw plunge, here) is built into the tool holder and apparently (from a brochure photo and description) activated by a hydraulic hose. So you could use a hydraulic cycle driven by the DSMA's control, instead of a ram motion, to give a sliding toolholder its secondary motion.