It is a box designed for electrical wiring. I consider it OK as long as you can't open the cover by the hinges, and that you consider derating if you run all the circuits in one large conduit between the two.
Sometimes it's a more attractive installation to have some 4X4 junction boxes over the old panel for the short wires, and pull the long ones into the new panel.
Many inpspectors here will allow a panel to be used as only if you screw the door shut, so wiring isn't exposed throught the breaker KO's. They won't accept breaker KO'seals/blanks as many easily fall out. Some will go as far as to have you write on the panel (Now j-box), "No breakers or fuses inside."
Mark Heller "Well - I oughta....." -Jackie Gleason
Then would that leave a hinge-cover wireway or gutter in the same boat? I can understand this in the case of a panel that doesn't have any door on it. The marking is a good idea, and I've screwed many of the doors shut myself. I just didn't understand the logic behind its being a requirement. Thanks
I have been reading the code book for 30 years and haven't yet found any requirement to screw the cover shut when turning it into a j-box. Now you could find lots of codes that pertain to other things that could be redily misaplied to this such as listing requirements and protection from physical damage, etc, but it realy is not prohibited in any way.