Hello Ben,
You can create geometry A using a Mechanical Model system, import the dynain (geometry B) using an External Model system, and combine them together in an LS-DYNA system:

Note that you will have to rename the dynain file to dynain.k to be able to import it using the External Model system.
Have you tried the above? I am using Ansys WB 2023R1 and the above workflow works.
Also, make sure to set the mesh preference to "Explicit" in the Mechanical Model system:

Let me know how it goes.
Reno.