inp file is a text format file, but you have to know the its configuration in order for SWMM to read it correctly. You can either figure it out yourself or buy a software that does it for you. We have a software called SWMM CONNECT GIS that interfaces SWMM 5 and ESRI ArcGIS 9.1. Take a look at it . . .
http://www.aqualyzeinc.com
RH