I would avoid a roof rack! the roof is only plastic and wouldn't support much weight, unless it is designed to only load the weight on the outside edges and it was in contact with the sides on the trideon cell. I doubt if there is an aftermarket rack that is made for a smart, but it is your car...
So far as I know, the only roof racks for our cars still in production are from Yakima. They mount to the tridion cell without bolts (so your roof and paint are safe) and is compatible with both 451 and 450.
I've read that some folks over at clubsmartcar have bought things like Thule racks then modified them to work with the fortwo.
I have a roof rack from my Smart Fortwo but I only used it to hold flags for a parade. I put them on at the beginning of the parade and take them off at the end. It is wider than my car; therefore, I would not keep it on longer than needed. I keep think I could hit my head on it (one day to shorten the length so it won't happen).
I have a hitch on my car too but it is was only intended for a bike rack. I don't use it often until I am able to attach a bike to it in a way that keeps from coming off and/or touching the ground.
During one parade that I drove it in - with military flags on it during a memorial day parade, I heard from the side someone saying 'my god, I thought my car was small'.