I'd like to make sure I have your intended usage straight.   You want to use MIDI output of the MODX to control the Launchpad X so the Launchpad plays its own drum sounds using the MODX as a master controller while the Launchpad X has only a USB interface.

If this is a correct interpretation, then I can say that this cannot be done directly.  At least I'm not aware the Launchpad X will act as a proper USB host like a PC would.  

What I know would be a working solution would be to add a tablet (with a USB hub) or computer with multiple USB ports and connect both the MODX and Launchpad X to this computer (laptop, MS surface type device, iPad, etc) aka "host" device.  This host device would be responsible for routing the MIDI and, if you wish, the audio.  I know the MODX more so I can say you could absolutely route the audio from the Launchpad X to the MODX and have all audio output come from the MODX using a knob on the MODX to adjust the relative balance of the Launchpad vs MODX.  Also allowing for you to add MODX's effects to the Launchpad if you wanted to do that (or just keep them dry).

With both connected to the host device, a DAW or some (minimally) MIDI router software would direct the MIDI from one device to other and MIDI channel configurations would make this all work together.   More details may be needed if you're not familiar with some basics of both MIDI devices - but this is a general "top level" description of how to accomplish what I want.  Assuming I have the usage correct from the onset.

If not.  If, say, you want to use the Launchpad X's pads to trigger MODX drum sounds then this could be done without a computer/tablet between.

The main limiting factor here is the way USB TO DEVICE port MIDI works on the MODX.  It's designed to have MIDI come in from the external device but not output MIDI to target that device.

SOME devices you connect directly to MODX's USB TO HOST port are themselves much like PCs so there may be a rare case where a device can be directly connected and "magically" (luckily) it would work for what you want to do.   This would put the onus on the Launchpad and how it works.  And this would most certainly be in the more "you're on your own" territory.