Yep, using a usb midi interface would be fine for MIDI in. (The usb-midi cables with a bulge in them are, basically, 1-in 1-out interfaces in a cable format).
The issue is not about MIDI but about USB: MIDI has no hosts/devices, it’s pretty dumb. But USB very much has a host/client structure, and norns is a MIDI host and that’s that. So you can’t just connect another host - your computer - to it directly. The 2host is a pair of clients that talk to one another, and thus connect 2 hosts.
But you’d equally be fine using a hardware MIDI cable going into a MIDI interface on the norns. So, in your BSP example, the BSP midi goes into the splitter - I presume a hardware THRU-type splitter - and then you take one cable into the IN of a USB midi interface/cable on norns. Then just make sure that the norns patch in question is receiving on the correct midi channel.