I would say yes. The sand in bulk form will not have a great deal of stiffness but the added mass will have a big impact on the frequencies. Since it is steel on the outside the stiffness contribution of the sand will be small.
As with any engineering problem, your first step is to evaluate (and then re-evaluate) your assumptions.
You're right that sand isn't an "engineering material" as far as SW Sim is concerned.
HOWEVER, consider the rate of application of your load. If your load is being applied slowly enough that the sand will mostly stay in the same place, then you probably don't need to worry about it too much. You can make an assumption on the density of the sand (for weight purposes), give it a high elastic modulus (as the mass won't deform) and model it in whatever shape fills the area that it's in.
But, if you think that sand will redistribute as you apply your load, then maybe this is a factor that will be highly challenging to account for in your analysis. Maybe you could model it as spring constraints, maybe you could try to model it with SW Flow Sim and assume that it'll behave like a highly viscous fluid (which... it won't), but you might have to be creative if you think you can account for it.
Thanks guys, i really appreciate your help. Today i will start analysis taking into account your suggestions and if you don't mind, will come back to you with final results. thanks again