I don't have BV, and I don't make default replacements, so I'm having to guess here...

If you open the Objects.package from BV, can you find the original suitcase in it anywhere? It seems to me that finding the original suitcase would be the best place to start. You are going to need the GUID and the BHAVs from it for your default replacement if it turns out that it is possible to make one.

I don't know the procedure to do it on purpose, but considering the number of times I've seen complaints about custom careers overwriting the Science career I suspect you would do it by cloning the original suitcase, copying the mesh and texture of the new one onto the clone without changing the GUID of the clone, and saving the new package.  

If that doesn't work, you would have better luck asking at MTS. There are more people who would be likely to see the question there.

It probably won't work, simply assigning a normal object to be an accessory.  For example, the accessory probably has an invisible marker on it to say which bit fits in the sim's hand.  A normal suitcase might go half way up their arm.

It's probably better to clone the accessory using object workshop, and untick all the cloning options boxes.  Do not at any time use "Fix integrity", and neither give it a new GUID.  This will ensure it creates an override.   Then edit the suitcase mesh in the package.   You can try replacing with your own mesh *in Milkshape* ensuring you export the mesh with the same mesh groups you exported it with.