Depends on what kind of wire the cable is made from. Aluminum cannot be soldered but can be welded under a tungsten inert gas shield (TIG welding). Copper is easily soldered without melting the copper. Both metals can be joined by mechanical pressure fittings, although copper-to-aluminum joints are subject to electrolytic corrosion because of the dissimilar metals.
or mount an IP65 rated box and join internally through suitable glands.
But a lot depends on what the cable is being used for (load), how long it is, whether it has RCD protection (being outdoors it really should have some) etc.