You don't "modify" something like that. Catia V4 is not parametric, so there's no way to modify. You need to re-create the face from the boundaries, using a planar fill.
Sorry - I assumed the face was planar, although in retrospect, I don't see how I got that.
Just do what 1oldcaduser and jackk suggested, and you'll be fine. If the face IS planar, then use Limit2 -> Create -> Surface. (this will create a planar face)
It doesn’t matter if it is planar or not, try to use fill command (SURF1-FILL). Select the close boundary (the hole), and then the surface. That command lets you to get a surface with a close boundary tangent to other.
kaeser - it DOES matter. He didn't say that he needed any boundary conditions. He says that he's *re-creating* a FACE.
If it already has a boundary condition, then there will be a surface as a parent. If not, it would be PLANAR, and therefore, have no boundary condition. And last but not least, the boundary condition may be handled by the adjacent surfaces, and thereby it may be either planar, or surfaic.