Hi,
i want to use one of these models for a new item:
clutter\briefcasedetonator\briefcasedetonator01.nif
or
clutter\chest\steamertrunk01.nif

These items are not reacting to physics by default, so how can i change this? Can this be done with the GECK or NifSkope?

I'm not sure but the item is probably static item. And static items don't use havok. You need to convert it.

How can i do that? Are there any tutorials out there for this?

Copy-paste it into a nif that does have havok.

Well it's not the real converting - you just change the item properties so it has the physics like any other object that can be picked up.
1. first thing you need to do is extract your desired mesh
2. in GECK open the menu of some object that you can pick up. It's the best to choose the most similar object.
3. and finally choose for this object your mesh - simply said: swap the meshes

Some more details and a pointer to a more detailed 3D art thread: http://www.thenexusforums.com/index.php?/topic/274988-activator-object-not-working (at newvegasnexus)

Also, do you find that other objects do react to the "run havok sim" button for you? Some people find it is not working for any object, and some of those people find that multibounds is the problem: http://www.gamesas.com/index.php?/topic/1148040-how-to-fix-lighting-havok-and-rendering-issues/

The Havok SIM does not work correctly in the NV-GECK, we're still hoping for a patch to the NV-GECK to fix this (and other things).

Until the GECK Power-up gets re-compiled against the latest patch (not so simple), we can only use NVSE correct (I find).

If your still stuck, I can convert 1 of the models for you.

Miax