One can easily convert basic Gerber (274-D) to Extended Gerber (274-X) using ViewMate software.
First, one needs to import the basic Gerber file. This can be done with "File/Import/Gerber", "File/Import/Guess" or "File/Import/Directory". The directory import is useful when there are aperture files that need to be imported together with the Gerber files.
After importing you may need to map the D Codes to the desired shapes (if aperture files were not present). This is done via the "Setup/D Codes" dialog. You are then ready to export the 274-X file (which includes aperture shape info) using "File/Export/Gerber Extended".