You can certainly fortify an intentionally sweet wine in order to achieve a port-like quality, but it's very difficult to find the proper "brandy" to make an authentic port. The traditional method entails adding a 100+ proof distillate. See Aguardente. To nitpick further, if your recipe uses fruit other than grapes, it's not really port, but that's a bit beyond the scope of the discussion.
More on topic, you'll need to make an extremely sweet/sugar rich must. When it's time to fortify (before fermentation is complete), you'll want to ensure that you overwhelm the yeast with alcohol so that they are unable to further ferment the wine and dry it out. Your final ABV should be above 18%.