As you know, MIDI is just data. (It’s a communications protocol for reporting events such as “Note On”, using packets that travel at the speed of a 56k modem.) These MIDI events could also be thought of as commands. So who’s giving commands, who’s passing them along, and who’s carrying them out?
Controller - issues commands
The Novation Impulse is a MIDI keyboard controller. It translates human actions into MIDI commands, and sends them on (via USB cable).
Sequencer - records and plays back commands
These commands can be stored, and later played back, so long as the relative timing is recorded by whatever “device” received the commands. Such a recording device is called a sequencer. Cubase has sequencers built in. A MIDI track is essentially a sequencer. I don’t know if the Impulse has a sequencer built into it or not. (Maybe it does, so it can store loops. I never use such things, so I don’t even know if my Novation SLMkII has any sequencers. My old Yamama YS200 certainly did.) Anyway, sequencers need some sort of clock signal, some kind of trigger, some kind of beat to measure time. The rate of the clock signal used during recording need not match the rate used during playback.
Interface - passes on commands
Now, the UR22 just passes MIDI along. The Novation can pass along MIDI commands too. Neither device can carry out MIDI commands that need to be interpreted as sound.
Synthesizer - obeys commands
A device that translates MIDI commands into digital audio is called a synthesizer. Many keyboards are combination synthesizers and MIDI controllers – they can issue commands, but also carry them out. And they can carry out their own commands. However, the Novation Impulse isn’t that kind of keyboard. It lacks a synthesizer on purpose.
So you need a synthesizer, either a hardware device or a software simulation of it (a softsynth). (Softsynths are cheaper and can’t be smashed by accident.) When you use a Hammond B3 software module, that must be the softsynth.
Back to your issue
Yes, you need a synth to obey the commands being sent from Logic’s sequencer. But it could be a softsynth.
No, the UR22’s main purpose is analog to digital and digital to analog conversions. It leaves MIDI alone, just passing it on.
You need to send MIDI from the sequencer (e.g. Cubase MIDI track) to a softsynth (such as Hammond B3 – assuming it’s a VSTi) to get digital audio, then route the digital audio to the UR22 (probably by routing the track to Stereo Out). The easiest way to bind a softsynth to a MIDI sequence in Cubase is to use an Instrument track. MIDI can be copied readily between Instrument tracks and MIDI tracks. (I don’t know how they do it in Logic, because I only have enough life to learn one DAW, and it’s not going to be one that runs exclusively on overpriced hardware.)
Now if the softsynth can’t be loaded into an instrument track, then I don’t know how you’re able to use it, unless it’s a stand-alone. In any case, you’d then be trying to send MIDI between applications, which is among those things I’ve successfully avoided so far. Maybe ReWire would help?