Connect your Keystep MIDI out to your Drumstation MIDI in. Set the Keystep to the same MIDI channel as the Drumstation (or vice-versa). Play the keyboard and listen to what instrument is assigned to which note. Press record. Hit eight notes. Press stop then press play. Listen to what it did. You just made a drum sequence of 8 steps in length. Hit record while the sequence plays and over-dub new notes.
As the Keystep is designed to be a chord sequencer, using to sequence drums isn't the most instinctive, but you'll get the hang of it. My first experience at programming drums was on a Korg M1 workstation synth with no drum pads, so programming drums was a very similar experience to using a Keystep, albeit the M1 has a screen (wow, the first time I've actually praised that 2line/32char display in over a decade!)
The auto-step behaviour of the Keystep in step-record mode is exactly like the M1's step-record mode and actually very useful for both notes/chords and percussion.
I just had a thought; I wonder what an Arturia sequencing workstation would be like? Sorta a KeyLab with a BSP layout above the keys, only with an extra SEQ panel for the chord sequencer.....