Question: I've got a situation where I want to put a wire in a concrete wall (I have a channel), but the one coming in isn't long enough. I cannot replace the incoming cable, so I need to join it with another one inside the channel. Said channel will afterwards be filled with cement, so I won't be able to access the joint anymore without some serious destruction. What is the best way to join the cable so that I can sleep easily knowing that there will be no problems there? If it matters, the cable will not be carrying significant loads (less than 100W max).

**Added:** Clarification was asked in comments. Location in world: Latvia. We're a 220V country. The incoming cable in question is marked "2x1,0" which (if I understand correctly) means two wires, each 1mm2 cross-section. Copper. It's pretty thin. The cable with which I will join it will be similar.

It will feed one LED lighting fixture (up to 50W, but probably less) and a wifi router. The location makes it unlikely that heavy loads would be attached there, however, of course, I cannot predict the future.

Answer: 
if i interpret your description properly - where your existing incoming wire length ends up ending somewhere within the existing wall, you pull that length of it out and cut it and have the splice on the outside of the wall. Then you get whatever proper wire and lay a single run of wire through the wall (channel) and have any splices happen outside the wall. So you would be wasting the existing length of incoming wire that goes however far into your channel. You want one good continuous length of wire that would be encased within the concrete wall never to be touched again.

in the US there is UF-B wire which has extra thick insulation and is rated for direct burial in the earth as well as being ultraviolet resistant. But it is not approved for being used in poured concrete. You would not want to run that or any other non conduit protected wiring within poured cement (or concrete) because as the cement cures is becomes very alkaline like PH 13 for a period of time, and that can harm the insulation of the wiring.... I mean you could do it and take your chances but to do it properly and have no future problems is why you would use some sort of pvc or metal type conduit that encases the wire and have the concrete/cement poured over that. And then after the wall is cured, you can always simply pull wiring through the conduit... you didn't elaborate but I assume your channel is a straight run; if not then you want it to be unless you have good reason for it not to be.
"cable in a concrete wall" is not a good idea, and this is why. No maintainability. Better to put the wires through **conduit**. This is one place where steel conduit is not good, and plastic works better.

With conduit, you can pull the wires out readily and at will, and then pull in replacement wires. The conduit must be built "to be able to be pulled" e.g. you can't have a plumbing elbow in there, it needs to be broad "sweeps" of turns. In North America conduit is required to be built empty of wires, and then wires pulled in after completion. This "keeps you honest" lol.