You do not say where you are located so I can not answer to your specific question directly. Except to say I have never seen this done in the USA and with good reason. I do believe it is against code to do it - but I am not a NFPA Electrical Code expert - and particular states might vary.
I am assuming you want to change out the panel with a newer panel and simply run wires from new panel to old panel and wire nut inside (maintaining your existing wiring). BAD IDEA. I would recommend that you NOT do it.
Change the panel out in place - remove your old panel and install the new one.
Putting the new one 15 feet form the existing - why? can be all kinds of reasons - but if you do not need to do this - Don't - all of your wiring drops to the existing panel area and will be much easier installing the new panel.
As others have pointed out using the panel is permissible : However it is only permissible under certain conditions (and one not listed here is that the panel be screwed shut or locked shut:
From the NFPA
[I think the 2011 code states this in 312.8] but I could be wrong on that.
Sec. 373-8 states you shall not use enclosures for overcurrent devices as junction boxes or raceways—unless you provide adequate space. Conductors inside a panelboard shall not fill the wiring space at any cross section to more than 40%, and splices and taps shall not fill the wiring space at any cross section to more than 75%.
Just to be clear as I do not know who in the future will read this i.e newbie DIY'er : Enclosures for Overcurrent devices ; an Overcurrent device is a FUSE or a Circuit Breaker.
Enclosures in the above sentence would mean something such as a service panel that has circuit breakers in it, often referred to as a Breaker Panel, Breaker Enclosure etc..