There are a few ways to go about this if I understand your problem but I'll give a general answer,.. First, the easiest way is to just import the file and not be concerned with the planes or coordinate system. Usually there is something on the surface or the boundaries edges or vertexes which you can refer to? Since you mentioned normal to surface want, what you can do is, apply some 3dsketch lines onto the surface to give some general vectors for how the surface will be oriented. Such as, apply the endpoint of a 3dsketch line coincident and tangent to the nonplanar surface (x vector), then apply another 3dsketch line perpendicular the first line and tangent to the surface (y vector) and/or add another, if you prefer, line perpendicular to both lines, the z vector or a construction triad, xys vectors. With those vectors, you can dynamically move the lines to establish some orientation and to help with setting up a plane or a new coordinate system. You can then use the above reference vectors or assemble the imported iges file. btw, the above is if you are re-importing that iges file often (that is, it's a client file and you do not have control of that data and the client shares/updates that file), keep the above geometry for future reference orientation/placement.
Otherwise, I'd suggest cleaning it up, that is, export the surface out as a parasolid (x_t) using the new coordinate system and re-import so the surface is set or centered on the default xyz planes.
..