How can I define CONM2 elements (Hypermesh) in order to obtain a more realistic vehicle frame behavior ?

Hi. I'm working on a vehicle frame. I supposed mechanical connections (between the tubes) as rigid elements (rbe2). But I wanted to take into account the mass of these connections. Rigid elements don't have any mass so I created concentrated mass on the independent nodes. In that way, I thought CONM2 elements were connected to the structure. But, I didn't notice any change on frame behavior (stress and displacement). 

I tried to add inertia properties (Card Edit>>elements>>Edit>>CONT), but it failed. 

I'd like to add masses that are taken into account in the post processing results... 

Thank you in advance