There is this USB host to Midi box for 45 Euro: http://www.hobbytronics.co.uk/usb-midi-converter
However it only supports one USB device connected. Might be perfectly enough for one midi controller, but you might wanna add more later.
There is also this for 65 Euro, which supports 8 devices via a USB hub: http://compasflamenco.com/usb-host-midi-p-6.html
Connect a Launchcontrol XL https://novationmusic.de/launch/launch-control-xl
And you will be happy 
It supports different pages of midi cc parameters, color coded knobs, etc…
Or wait for this custom box: New FADERFOX MIDI controller system for ELEKTRON devices (12-track-design for ANALOG RYTM) It’s more tailored for rytm but should be editable for Digitakt too.