As far as I know any Gerber data will only contain physical structures- which will not have any electrical information. Hence with Gerber- one cannot reconstruct the PCB CAD. Coming to importing .PHO to expedition- no such option is available AFAIK.
The filenames you see when you import gerber data are the filenames that are defined in de gerber settings for Expedition. (Gerber Preferences File, .gpf)
You can either change the names of the gerber output files "Output -> Gerber" or just rename your files.
As 274X is a standard, the file extension should not matter at all.