It's simple

Open the object you want to edit, in the CK. Click EDIT next to its MODEL. You will see a filepath similar to the following (this is a Wolf Pelt misc object):
Clutter\WolfPelts\WolfPelt.nif
Copy this filepath exactly (drag-select, then right-click & choose copy). Now create your new object. For it's model, click EDIT. The next window which displays the nif file, click EDIT below Model File Name. It'll open up your Windows file manager. Simply browse to the main MESHES directory, and paste your copied filepath into the File Name box like: http://i1159.photobucket.com/albums/p628/B1gBadDaddy/MeshFilepath-1.png
Clutter\WolfPelts\WolfPelt.nif
Click OK. It'll then all tally up properly, showing your chosen NIF file, as the nif for your new item. No need to extract BSA files either, the CK will automatically look at your BSA.
Job done. Who's the daddy?
EDIT:
But just so you know, using a STATIC mesh file might not give you a proper "havokable" object. Statics, even when you use the NIF for another type of item, tend to remain STATIC when placed in the world. But it's good for creating misc objects from food items, or weapons etc.
EDIT 2:
Obviously some objects have a couple of different texture options, like the above Wolf Pelt. You can change those by double-clicking the name below 3D name in the window you can view the item, and select the appropriate texture. Wolf has normal black, and Ice (which is the default texture). Not all objects have this, just food for thought!!!