The procedure of using IWADS as PWADS is a technique done by savvy players of DOOM, and its games. For instance, one way to getting Chex Quest to represent its levels with DOOM resources is to treat DOOM2.WAD as a PWAD. However, one disadvantage to this practice is crashing some Doom engine interpreters. Even Chex Quest itself is an IWAD that uses its own executable (later on internal source port support) to start up, in which its WAD has the PWAD megawad feel to some players.
The practice of treating IWADs as PWADs actually dates back to when some IWADs were only in their developmental stage. Files that would actually become part of an IWAD (such as levels, graphics, etc.) have actually started out as PWADs for developers to test with. Even Doom itself started out with candidate PWADs of levels.