Stream-lined Civ V conversion process:
If using CivNexus6 1.2.3 your Asset .ast file will be created automatically as part of the "Convert all .gr2 Files in Directory to .fgx/.anm" step. This works by assigning Civ V animations into the Civ VI animation slots. The mapping is editable if you need to tweak it - see the Civ6ToCiv5AninMap.txt in the CivNexus6 application directory. Another file containing just the Behavior data is generated within the fgx_anm_output folder called m_BehaviorData.xml in case you want to copy it into an .ast file manually.
The Asset file should allow the unit to work in the Asset Previewer and Civ VI once you have hooked up the Materials and Textures. There will be no Attachment Points, Sound FX or Visual FX included initially so you will have to edit these in yourself.
As with the CivNexus6-generated .geo file, the .ast file has no indentation formatting, but this is easily fixed in Modbuddy after you import it using Edit > Advanced > Format Document (shortcut Ctrl-K followed by Ctrl-F).
For more info see the Civ V conversion tutorial thread.
3D Graphics Tool for Civilization 6