How do I fill my grill's propane tank from my house tank?
When I asked our builder and the gas supplier about this, I was told unequivocally no. None of them wanted to get involved in any way in allowing a homeowner to be able to fill a propane tank.
I was told that this was a &quot;safety issue&quot;. As has been typical, the people I was actually talking to don't know the code chapter and verse and so could not of course document any actual legal prohibition. But they were adamant that there was in fact a legal prohibition, and that being able to fill a propane tank at home was a non-starter.
Even if it turns out that their information isn't perfectly accurate, it's clear that filling a propane tank is at least a little finicky, and you will want to make sure that the equipment is installed by a professional, and that you are given adequate training not only on the process, but how to deal with things if/when the equipment doesn't work as expected.
Some quick research online, and I can add to the above:

At the very least, it is likely you will need your bulk propane tank retrofitted. Filling a grill tank (typically a 20 lb tank) requires liquid propane. While your bulk tank has liquid propane in it, the propane that comes from it for domestic use is the gaseous propane at the top of the tank. You would need a separate &quot;liquid propane tap&quot;, which involves connecting to a different line attached to the tank that pulls from the bottom of the tank. It seems that while it is common for this line to be present, it normally is not configured to be usable by the homeowner. Your gas supplier would need to attach appropriate hardware to the tank to allow its use. Which brings us back to the point above.

(Aside: you need to fill with liquid propane, because just as evaporating the liquid propane reduces the temperature, allowing frost to accumulate on the outside of the tank, compressing the gaseous propane enough to condense it will heat it up. Even if you had a pump available capable of compressing the gaseous propane that you get out of your bulk tank, to fit it into the grill tank over any reasonably short period of time you'd wind up heating the propane up enough to be dangerous.)
The bottom line is: if you can find a gas company that is willing to help, this is possible. But it's not DIY, at least not to get things set up initially.
There is, no surprise, a lot of conflicting information to be found online. IMHO, a lot of it comes down to just how willing you are to risk blowing yourself up. FWIW, I found this post one of the more useful and apparently well-informed.
My favorite quote from that thread, regarding the wisdom of installing the necessary adapter equipment on your bulk propane tank yourself:

if you insist on doing it yourself, get you family to go out for the day, move your car or truck at least 100 yards away and leave you wallet and a note in your vehicle. It will help the emergency response folk understand what happened if there's an accident.

You may also find some useful discussion here: https://www.tractorbynet.com/forums/rural-living/137881-20lb-propane-tanks-can-you.html. As always, take everything you read with a grain of salt. That's especially important here; looking through these threads and others, it's clear a number of people have successfully filled their own propane tanks without killing themselves, only due to them having remarkably competent guardian angels.

For what it's worth, what you _can_ do (i.e. what my gas supplier has done) is provide a quick-release tap near the grilling area, so that the propane grill can be connected directly to the household supply.
Note: this may require some tweaking of the gas regulator(s). Weber, for example, requires the customer to contact them directly to obtain the hook-up kit. I have read in their customer reviews (though didn't see anything in their official documentation) that propane grills require 14 water column-inches of pressure, while normal household pressure is 11 wc. This would require a separate regulator for the gas hookup than for the other household taps, something that the gas supplier would need to provide for when installing the tap.
Update: Since writing the previous paragraph, I have acquired a Weber grill and the necessary parts to connect it to our household bulk propane. Note that I already had the gas installers provide a pipe with a shutoff valve at the barbeque area. To order the Weber parts for the hookup, I was required to provide them with the serial number for my grill. Even with that, the parts included only the two mating parts of a quick-release connector, along with the hose and regulator to connect to the grill. I still needed to purchase the necessary adapter fittings to go from the quick-release connector to my gas line (if I recall correctly, the Weber parts had a 3/8&quot; threaded connector while the gas line was 1/2&quot;).
The gas pressure turned out to not be an issue. The Weber parts are designed to work correctly with the standard household pressure of 11 water-column inches.