Adding a wire or circuit to a disconnect box
Current code would require 4 wires for the sub panel. The 40 amp circuit should have at least #8 wire. With a 40 amp breaker in the main you could feed a 60a sub panel with 4 wires and meet code, if this panel is outside you will need an outdoor panel or a NEMA 3r enclosure. Also the neutral buss needs to be isolated from ground. The ground buss still connects to the case but neutral and ground can only be tied together in the main panel.