There are a couple of things you should check first, and I'd not use the sweet mead yeast if you can get your hands on some Lallemand active dry such as K1V-1116 or EC-1118. Both the White Labs and Wyeast sweet mead yeast strains are notoriously flaky -- and they are the ones least likely to re-start a stuck fermentation. If D47 stalled out, then you'll need a more aggressive strain to complete the job.
Before you do anything, I'd check the pH of your brew first. Three things are the most likely causes of your stuck fermentation, and of them, the very most likely candidate is too low of a pH. You want it to be above about 3.3, and below 4.0, ideally. The second most likely cause is insufficient nutrient and/or oxygen at the start of fermentation. Since yours is a braggot and the grains provide nutrients that honey lacks, insufficient oxygenation might be more likely the issue. The third most likely issue is with possible improper rehydration of your yeast at the outset. If you have not read hightest's Mead FAQ pages (top of this forum section), I suggest that you do now; they'll give you much more insight into what may have gone wrong, and what you can try to do about it, than I can in a few short paragraphs here.
But after you've checked to ensure that the pH is no problem, then yes, you will need to make a starter with any yeast that you are attempting a re-start with. You will need to acclimate that starter to the environment currently in your main batch by adding small amounts of the main batch to that starter over the course of a day or more -- then you'll be ready to pitch. If you choose to add nutrients, only use those that contain no DAP at this point. Yeast can only use amino (organic) sources of nitrogen this late in fermentation, with the ethanol level where it already is in your main batch. If you can't get anything like Fermaid-O or Fermaid-2133 (both are amino-only nutrients), then you should add some "yeast hulls" or "yeast ghosts." That is basically dead yeast, which will both add some nutrient value and also introduce broken up yeast cell walls, which will serve as a good toxin-trapper in your main must.
Good luck - restarting a stuck mead or wine is a lot harder than doing a beer. Braggot will be somewhere in-between, but it still takes care on your part to maximize your chances of success.
