Motion Builder - exporting an animation to an .mb
Using ascottk's rigs.
First, load the AnimRef_idle.fbx of the character you want, into Motion Builder, and animate with that.
Before exporting, you have to do this ritual; up on the top right where the picture of the man is, there are some drop down menus... Click on "Edit", then "Plot Character", then click "Skeleton", then "Plot".
Then click File, export, and from the file-type selector, choose "Kaydara FBX (Animation only)". Like it says, this will just be the animation only, and no cameras or models you may have in the scene.
In Maya, open the fbx_import.mb. Then go File, Import, and choose your FBX file. You may need to install the latest FBX importer plugin. Choose the Merge(Exclusive Merge) option on the importer dialogue.
The animation should play correctly and you should be able to save this as walk.mb or whatever animation name you want, and then use the usual Doom 3 export process to get it into Doom 3.