Freestyla
Are you talking straight out unhopped extract cans, or actual "kit" cans, like the ones you get at home brew shops, or the supermarket? If you are talking about kit cans, like a Coopers Draught or something, then yes, you can just dump some hop pellets into the fermenter, and it will add some hop flavour, and more than likely improve the kit. This (as pointed out earlier) is known as "dry hopping". Another method you can use is the "hop tea", where you can get a French Press, stick 30g of hops in there (I would suggest some hallertauer, or tettnang if you are doing a basic kit, saaz if you are doing a lager kit, or something american like cascade or amarillo for an american style kit) and boil the kettle. Fill the hopped french press with the boiled water, maybe even let it cool 5 mins in the kettle first, then give it 10 mins in the french press, then press the strainer down, and add that to your fermenter with the kit. This should give a bit of hop character too, and improve the kit. Ask your local home Brew Shop for advice. Or ask here!
If you are talking about the unhopped extract, then you need to boil this stuff to add bitterness. You DO NOT need to steep cracked grains in this type of beer, but it can help you achieve the hallmarks of a certain style (such as the use of crystal malts in an american pale ale, or english bitter). When I did extracts, I found that I could use 3kg of unhopped malt extract in a beer to get me around the 1050 mark. I would boil about .5 to 1L of the extract in about 5 or 6L of water for an hour. I would get the bittering hops, tie them up in a stocking piece (with heaps of room - they expand alot) and add them at 60 or 45 mins, whatever the recipe I was using called for. The I would do the same for any flavour hops (30-15 mins) and aroma hops (15 mins to steeping after I shut off the boil). After boiling, you dissolve the rest of the extract in the (now very hot) water, and add to your fermenter, top it up to 23L with cold water, and pitch the yeast. I used to fill up a 15L jerrycan and stick it in the fridge for a day before I did an extract brew, so when I added it to my boiled portion, it would end up around 20C in the fermenter, and I could pitch my yeast straight away.
I think it is very smart of you to want to learn the basics first, but generally all grain and partial brewers know how much better they feel the additions of grains and hops have made their beer. Pretty much all of them have started on kits, and made their way up from there. A fellow all grain brewer (and a brewer who I really respect) recently told a new brewer to follow the instructions on the can first, then start making little tweaks from there. That way you will get a feel for what each little change you have made will make to a beer. If you start out steeping grains and adding hops and you dont like the beer, how will you know what part of the process made the flavour the way it is? Keep it simple, take little steps, and soon enough, you will know heaps about hops, grains, etc... and probably spend way too much time brewing, or thinking about brewing liek the rest of us!
All the best
Trent