How do I convert a service panel to a junction box?

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.

EDIT 4/17/2017 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..

Assuming it's not prohibited by your local codes and the existing panel is not violating any current codes like overfill, working space clearance, there's nothing code-wise that says "No this is prohibited".

However, a better solution if possible, since you have to run new wires anyway, is to install a 12x12 or larger junction box in the attic and crawl space and be done with it.

Now it would be wise to consult you Building and Planning Department first as most of the time the Power Co. will not reinstate power unless the work is permitted. Also, not sure about your local jurisdiction but around here they make us install hard-wired smoke alarms in every bedroom and in every hall near a bedroom if a panel is upgraded.