Although I have not much experience with such an analysis, I think that I-DEAS is not capable of it. I assume that you want to simulate how the empty balloon is filled with gas and subsequently gets larger until it is completely filled. I see the following obstacles for I-DEAS:
1. You'll probably have to consider special material laws for the balloon skin, they're probably not available in I-DEAS
2. I-DEAS cannot cover large displacements which occur during the inflation of the balloon
3. Depending on what you want to analyse, you'll have to perform a CFD study in order to determine the velocity/pressure field of the gas filling. In the worst, You end up with a coupled flow+structural analysis (gas + ballon skin)
However, you could use I-DEAS as preprocessor, i.e. generate geometry, mesh the whole thing, apply boundary conditions and export the FE-model to your chosen analysis tool. If you're concerned with 1. and 2., one of the "big players" in FE analysis (ANSYS, NASTRAN, ABAQUS) should do.
Is there any CAD software that allow soft body simulation and also being user friendly. It is not as much of a FEA as it is suposed to be an animation with real and natural forces and patameters.
I mean, here we are in a world full of soft bodies and we only are able to work with a fraction of those that are so called solids..!!!! I think thats strange. Engineerign science, with all its complexity is looking pretty primitive considering the previously mentioned things.