An IGES can not be modified, it can just be patched and tweaked (gaps closed, etc).
The only way to modify dims is to do it in the originating program that created the IGES.
If you brought the IGES in as a solid, you can add features as you need to. But don't listen to anyone who says that an IGES file could be made parametric using the "Warp" tool, becuase they have obviously never tried it for anything more complex than a simple block (I used to have to put out fires like this started by sales guys when I worked for a VAR).