I have been playing and reading the forum since a month after Valheim's release. I have never seen or heard of a way to automate feeding the smelter with either coal or ore.

A player is not, however, limited to one kiln or one smelter per base. Provided one has a decent supply of sertling cores, one can set up three or four kilns and produce a lot of coal. After making enough coal, tear down the kilns and replace them with smelters. Load up all the smelters and go do something else. I usually farm or craft foods. When the first round of smelting is done, reload all the smelters. Repeat until all the ore is processed.

Refining all one's ores may take two or three days with one smelter. Multiple smelters reduce this time significantly.