If you have springs in the system they should be in the model as well - statics or dynamics. If you only wish to add stiffness, then a restraint will do the job - simply add the stiffness in the stiffness field (a blank stiffness indicates rigid). If you wish to better model a hanger support, the spring preload (theoretical cold load)should be included. To do this, define a hanger rather than a restraint. The bottom of the hanger input window allows you to set the stiffness AND preload.
Without good load information (to define the cyclic mechanical vibration of the engine on your exhaust) the most you should expect out of the program is the possibility of exhaust resonance with the engine. Do this by simply calculating the natural frequencies of the exhaust. If the exhaust frequencies are close to the cyclic speed of you engine, then you can expect trouble. How close is close? Some would say within 20%. Others may also be interested in multiples of the running speed.