I'm faced with moving a breaker panel about 6-10 ft. Reason is that previous owners removed much of a termite eaten load bearing beam and posts and the house has started to sag. This looks to have happened 80-100 years ago. Then it was covered over and the breaker panel was installed to one of the rotted posts. The breaker box has to move to make room for the new posts and beam and remain in it's new location. From the meter there is a very hefty cable of romex type inside the wall about 25ft long from meter and when the cable exits the wall it is inside a hefty metal conduit then connected to the breaker box. Is it OK to put a junction box and splice another 8-12 ft onto this main feeder cable to reach the new box connection and put this wire inside the wall ?
What size junction box ?

In this very old house the wiring is BX and the insulation is crumbling, so I intend to pull new romex from the breaker box as much as possible and use ARC type breakers, for when I cannot replace the BX. The BX insulation is crumbling perhaps as much as a foot from connection points. When I tested a few BX of this vintage approximately in the middle of the cable the insulation was in good condition. I have access to a lot of the wiring, since the middle of the house has been demo-ed and much is exposed.
I'm working with a contractor, but I'm doing much of the plumbing and electric myself.
Any input is welcome...
Thanks

The cable can be spliced, but to suggest the size box depends on the size cable.
Need to know wire size, quantity of conductors, amperage rating.

Where is the main breaker located, at the meter or in the panel?

The main is now probably in the panel, if so, once this cable is lengthen the main may need to be located at the meter.

Can you provide some pictures?