ebike charger workaround

[m3vuv]

hi all i have an 18s/8p ebike battery,its a colbalt pack not lipo4 so 4.2v per cell,i have a charger for a 13s pack,ie 54.6v the 18s pack needs 75.6v,can i set my lab psu too 20v and wire that in series with the 13s lipo charger and use that to charge the 18s pack?,thanks in advance,m3vuv.