Electrical – install a 50A breaker in a 70A sub-panel with other breakers installed

Yes, you can. The systematic computer programmer in me even thinks it's a good idea, allowing a "star" topology for electrical distribution.

1. You will need 4 wires, as others have mentioned.

2. Keep the ground and neutral separate in the subpanels. The only place they should be bonded is in the main panel.

3. You will need ground rods at the new location. Yes, it's redundant, but it's still required.

4. The wire feeding the new subpanel must be protected from overcurrent by an appropriately-sized breaker. It can be at either end - in the old subpanel or in the new subpanel - or at both ends. It's up to you, but if only using one, I slightly prefer the upstream location, so that I can easily kill power to the feeder wire. If using both, 1 must be sized for the conductor, and the other can be that size or larger -- convenient if you buy a panel with a 100A breaker preinstalled.

5. While there are wires rated for direct burial, it's good that you're planning to use conduit. Oversize the conduit for ease of pulling wires, and minimize bends. While you have the trench open, run a second conduit for future expansion (ethernet? gas? macerator?). May as well run a water line, too, so you can have a sink.

6. Most electric kitchen stoves are installed on 240V/50A circuits. If you're installing something like that, then I suggest a 60A subpanel. That gives you plenty of headroom to install lighting, a small point-of-use electric hot water heater, and some convenience receptacles. Those will come in handy when doing maintenance in the area.

7. Aluminum wire is usually much cheaper than copper, but harder to work with. It's stiffer. You must use 1 size larger than with copper. The ends must be protected from corrosion with anitoxidant goop. The lugs must be torqued correctly. Some people think you should retorque the lugs on some schedule. Last time I bought wire, I wanted 8ga copper, but 2ga aluminum was 1/2 the price, so going with Al was worth it.

I would start by putting a rather large subpanel as close as possible to it, probably whichever Siemens panel ThreePhaseEel recommends, or a CH or QO if space is an issue. 42+ space. Position it with an eye toward rerouting the service wiring there.

Even before that, talk to the power company about upgrade options. If a particular size is possible, get the subpanel with that main breaker, but don't sacrifice spaces to do it.

Short term, if the new subpanel is >=100A it doesn't need any main breaker at all, or even a main-panel breaker. The main breaker in the main panel will protect it. Use at least 100A wire to connect the panels, i.e. At least #3Cu or #1Al. A reason to use cheaper Al on such a short run is the lugs will be aluminum.

Having a breaker in the main panel can be avoided with a lug-breaker, which snaps in like a breaker but has only lugs.

Now, at leisure, move one circuit at a time from the old panel to the new. That's just to get away from those somewhat dodgy Challenger breakers. If wires don't reach, I typically put 4-11/16" square boxes at opportune locations to splice. Ideally metal conduit the rest of the way to the new panel, and THHN wire inside the conduit. Ideally.

With all the circuits moved over, try to straightline the subpanel straight off the bottom (switched side) of the main breaker.

Alternately, if the power company's upgrade path calls for a new meter pan, get one with a main breaker. Gut the old panel, using it only as a gutter, to the new subpanel, using wires rated for main breaker current. If that's the plan, you can use a main lug subpanel instead of a main breaker one, saving some space. 42-space main lug CHs are not that big.