You should be able to do it using
NifScope (which can be found via that page of useful programs for modding over in the modiki).
Find the NIF file (or files, if it has an FX version too - you'll want to make the same adjustment to both) for the unit by looking at the art define for it. The actual NIF has a good chance of being in one of the FPK files, which you can unpack with the PackBuild program on the same page.
Open it with NifScope. (See the attached image.)
Find one of the root nodes that the unit's "skin" is all attached to. I'm not sure, but in some cases you may need to adjust more than one. The top level nodes will tend to not have any effect. That is because the animation files specify positions and any information that is "before" (closer to the root of the tree structure of the nodes in the model) the place where the animation starts is canceled out by the positions specified by the animation (at least, that is what I think is happening when you make adjustments that have no effect like that - I'm not a modeling expert, but so far none of them have responded so here I am).
For the selected node, apply a translation to shift it in the negative Z direction. Since +Z is up, making it a lower number, or negative, will move it down. The values vary as the scales of the models vary some. In general 100 is a lot, and even a shift of 10 might be larger than you want. The blimp in the attached image was adjusted (by me) to 99 units, up from the original 0 in this node. Note that when a model is scaled in the art def it will scale this distance too (unless it is 0, since it is multiplicative).
Save the adjusted model. Put it in your mod in the location specified (or that will be specified) by your art def for the unit (probably something like "Assets/Art/Units/StuckInTheMud/thingy.nif").
If you are not overriding an existing definition, you may also need to copy the texture files too - those would be the .dds files - or it may not have the right textures. Point your new artdef at the new version of the model. Give the new unit XML the new art def.
If you are overriding an existing unit you can put it in the exact same path that the original was using. The model in the actual folders will override one in BtS's folders or the FPK file. IN this case it will still get the textures from where they already are so you should not need to copy them.
Fire up the game and open up the 'pedia to look at your newly adjusted unit.
That should be it.
Unless I forgot something or made a mistake...
