What I have done for several of my hollow Shapeways models to make them heavier is to add casting resin. I have used this successfully on the Shapeways SWF (Strong White Flexible) material. I inject this into the hollow interior of the parts.
I have used Crystal Clear 20 Casting Resin. This is a 2 part mixture that has equal parts A and B. The nice thing about this casting resin is that it stays thin for about 20 minutes so you have time to work with it. You can also try 2 part slow cure epoxy. The Casting Resin and Epoxy is available many places online. I have found the slow cure epoxy at hobby stores for RC cars and models.
If your model has an open bottom your can just pour your mixture into the bottom.
If your model does not have an open bottom then you will need TWO holes for injecting mixture inside the model. You can add holes when you create the model, or drill as needed. Make the holes at least 3mm in diameter. One hole is to inject the mixture into. The other hole is for venting air ( otherwise you get a mess ).
I have used Monoject 412 Curved Tip Syringe
, 12 cc for injecting the mixture. These are available online, or at many hobby shops. I cut off a half inch or so off the tip to make a bigger hole for the resin to pass through to make it easier to inject. Just make sure that the tip still fits inside one of your holes in the model.
I use small plastic disposable cups for the mixture. Pour some into the syringe. Insert tip of syringe into hole in model. Inject the mixture. Refill the syringe and inject more if needed.
Use a small square of non-porous rubber to cover the holes. Clamp this in place. The rubber makes a good seal, and can be peeled off easily after drying.
Next rotate the model so that the mixture flows evenly over all the interior surfaces of the hollow region of the model. Keep rotating until you reach the cure time. For example until 20 minutes since you mixed your resin. This will keep the mixture from running all to the bottom.
Let fully cure overnight. Remove the clamp and peel off the rubber seal. Your model is now ready for painting if desired.
Hope this helps.
Post your results here if you give this a try.