It depends on what you mean by "pre-foreclosure". People often refer to short sales as "pre-foreclosure" and yes, by all means you can make an offer at that point. However, if the home isn't on the market and is close to being foreclosed on, i.e. owners have missed many payments and the bank is going to file the "NED" (notice of election & demand) with the county, then it might be difficult to make an offer at this point. Once the bank has started the foreclosure process, you might as well wait for it to go through the process, and see if you can buy it if/when the bank buys it back and lists it with a Realtor.
I also like to warn people that buying short sales and foreclosures aren't always a great deal. It can be a complicated process (especially short sales), and you may not even get the property in the end. And it may not be that great of a deal. Most "distressed" properties have been neglected and need some work. There are some deals that are better than others.
When you're buying any type of home, whether it's a short sale, a foreclosure, or a regular home, you should find a good Realtor to work for you as a Buyer's Agent.Good luck!