Yeah the only way I could condone this would be if you were to completely tear down the battery to individual cells and rebuild from there.
Using the old 10s6p and adding 2s6p of new cells = no bueno
Tearing down the old 10s6p and reconfiguring into a 12s5p = all good as long as you don’t damage any cells
If you really must have 12s6p, the best way to do it would be to tear down to individual cells and essentially make 12s5p of old cells, and then add a new cell to each p group. Mixing old and new cells in parallel is many times less prone to incident than mixing them in series.
Please read my favourite thread on this forum for more information on mixing cells in parallel