kev - fruit flavors are relatively easy as most homebrew supply shops (at least in the USA) carry flavoring. I'm guessing that based on your mention of liter sized buckets and spelling of the word "flavour" you're not in the states, so that may be more of a challenge - you'll need to let me know if your favorite shop carries flavoring - if not an on-line supplier should have them. One thing I've learned in adding fruit flavors to beers (and I've only done a few) is that you really need to create a test sample to suit your tastes. Here's how I've done it. Start with four small samples of your beer that's fully fermented and ready to bottle. Add flavoring a drop at a time, counting the drops. Try four in one sample, eight in the next, twelve, then 16. Taste each one to determine what you like. If that's not enough flavor, add a few more drops until you (and maybe a few other taste testers) agree on the right flavor. The scale up your test sample to the full batch size. You can find information on-line calculator that will tell you how many drops in an ounce or ml. Then do the math to calculate how much flavoring to add to your bucket before bottling - it may not be a full bottle of the flavoring you purchased. You might want to do one more "full batch" taste test to insure the flavor is what you want in your finished beer. Keep in mind that you can add more flavoring, but you can't take it out of your beer - if you add too much, you'll likely regret it.
Smoke flavoring can be achieved in the same way by adding a liquid smoke product to your finished beer, or by including some smoked malt in your recipe. If you're using an extract kit, you could steep some smoked malt - but again this will be a little trial and error (hopefully not too much error!), and a little smoked malt goes a long way. And you really need to be careful with peat smoked malt if you decide to use it.
Another thing to try might be making your own extract. Use some dark toasted oak chips soaked for a couple of weeks in vodka, then add the liquid extract to your brew. I've done this with rum and oak chips to add a rum barrel flavor and the ale I brewed turned out rather nicely. I also added the oak chips to my keg in a nylon bag, but if you're bottling adding oak to the finished beer would be a little challenging!
Best of luck and let us know how it turns out.
