as you may already know, the Doom 3: Bfg Edition that is currently sold on Steam includes Doom 1 and Doom 2 as "Classic Doom". However, the included doom.wad and doom2.wad files are technically PWADs - that is, well, technically they are indeed IWADs but the first byte in these files is 'P' instead of 'I'. The most notable changes from the real classic IWADs are that both of them feature a DMENUPIC lump and that doom2.wad is missing the TITLEPIC lump (among others).
However, PrBoom+ currently fails to load them as IWADs, because they fail the CheckIWAD test - although it is theoretically possible to play them.
Please find attached a patch that enables PrBoom+ to use the "Classic Doom" WAD files bundled with the Doom 3: BFG Edition as IWADs. It softens the CheckIWAD test a bit to merely throw a warning if the IWAD header is not found instead of instantly bailing out. Then it checks if the WAD file in question contains the DMENUPIC lump (which is unique to the IWADs bundled with the BFG Edition) and continues if it can find it - otherwise it bails out here. Now, as the doom2.wad file misses the TITLEPIC lump, we have to replace it with another graphic lump in the demo loop. The new DMENUPIC lumps comes in handy, as it serves perfectly as a replacement and furthermore allows to distinguish the real classic IWADs from the new ones from the BFG Edition at first sight.
Hope that helps,