The risk of contamination is much lower after primary fermentation is complete, since there is alcohol present. The alcohol will prevent or retard the growth of bacteria and rogue yeasts. So, you should be safe to just add your adjuncts.
With some fruits (strawberries especially), I have noticed that the beer seems to spoil after a month or so, presumably from some rogue yeast on the berries that eventually took hold. The beer was great before that happened, so I'd just suggest drinking it quickly.
Remember, people have been brewing for centuries without knowledge of microorganisms, including yeast. I always remind myself of that fact when I start to worry too much about the technicalities of brewing. I also remind myself to "Relax, don't worry, have a homebrew!"