Can't tell exactly what you are trying to do. A screen shot of an attempt might help. Also, can not open in 4.0.
Looks like several portions of the edges have been extended already. Holding shift and selecting all of the edges to extend all at once usually works better. Extending an inset surface edge can be very problematic.
You should also pay attention to the extension method in the options tab.
Looks like you have an import feature that has been offset and you then started extending parts of the surface. This creates openings such as:
Not controlling extension method has created a jagged point:
As I mentioned before, when extending the surface, hold the shift key and select adjacent edges all the way around the surface to extend all of it together to eliminate these issues.
Also, being an imported surface, there may be too much "wobble" in the surfaces to extend it very far. I was not able to extend the imported step surfaces you supplied. (they were converted twice, Creo to step and step to creo, degrading at each step) You may be able to use the import data doctor to "fix" the surfaces. I have found that imported surfaces tend to be problematic especially when they are not planer or cylindrical.
