Hi Linuesa,
We didn't test the Keystep connected to the retrokits rk-002 midi cable. But on the retrokits web site it's stated that this cable works with Beatsteppro. So it should work on Keystep as its midi output is really similar to the Beatsteppro one.
That said, I don't know what is the power consumption of the retrokits rk-002 midi cable. So the limitation could then comes from the power source more than from the Keystep itself. For example, As the Keystep is a low power device (drawing less than 100mA) it can be powered by a portable device such as smartphone or tablet through USB. If the retrokits rk-002 midi cable is plugged to the Keystep, it will increase its consumption from the USB and maybe the total consumption (Keystep + retrokits rk-002) will be above the 100mA limit. So in case of "portable set-up" the total consumption of Keystep + retrokits rk- 002 midi cable must be less than 100mA.
If the Keystep is powered by a computer or by the DCin connector (stand alone), I think there will be no problem (as on Beatsteppro).
Best regards,
SCo