Use the "FeatuteWorks" feature in the Solidworks. I use it always when the suppliers/costumers give me step or iges file. It isn't available in the standard version. This is able to rebuild the model with Solidworks features. Therefore you can modify the new part.
(ehhh....I should write a tutorial for the challege ....XD)
Another name for these imported file types is dumb solid. The reason is because there is not a feature/history tree with sketches or dimensions to edit. You can, usually, add to and cut away the dumb solid to modify it. Just pick a planer face of the imported geometry if there is one, if not create a plane. Make your sketch and cut it or add to it. You should be able to do this with any 3D solid modeling program. If its complex surfaces it depends on the situation and what you want to do. Feel free to upload an example with your specific question.
In the above video I show you to edit "dumb" or rather imported files. The process is the same for virtully any file format. IronCAD is the king at this since it can generate editable features on-demand and not have to run through a whole "featureworks" approach that creates unruly parts. In addition it adds handles to each surface for easier editing as well as more geometric constraint editing ( like Spaceclaim promotes). That with direct face manipulation and the tradtional boolean approaches outlined in this post, IronCAD cannot be beat for ease and speed of editing "dumb" data. You can even assign parametrics to the dumb data if desired. So if you find you need this, I would strongly urge you to invest in IronCAD or the cheaper lighter version Inovate which does the same thing.
Here is a video tutorial. It's useful. But only to parts.
My experience, it isn't so good in Assemblies, which include plate and body parts too. The program (I mean the featureworks function) unable to choose the right function that I get a right body or plate part.