you can use the midithru to send the input to output
(or you could do this thru patching midi/in objects to out in a subpatch to filter, see below)
to 'filter' which source (din or usb) , you need to create a subpatch, and the on the patch settings select 'midi selector' , this allows you choose midi channels, and also midi source (din/ubs device/usb host) for input
personally, seems a faff though, why not write an arp in Axoloti... then you don't have to mess about sending midi to an external box
(there might be an arp in the community library, ive not looked...)