I need to substitute fresh yams for canned, but I want the sweetness
If you want to devote the time to get the maximum sweetness, you can take advantage of natural enzymes to accomplish your goal.  If you can  hold your yams between 135 and 170°F (57 and 77°C) for a couple of hours, you can dramatically increase the sweetness. They will still be hard at this point, so will need roasting to soften. A sous vide device will be the best way to do this, but you can do a decent job of improving sweetness by roasting, wrapped in foil, in a 300 F (149 C) oven for about 2 hours.  The science and process is explained in detail here.