Yes, you can do that. Many people do it all the time. Your hop utilization will lower, and your beer will be darker, but that something many brewers deal with quite successfully.
Like many others around here, I strongly suggest that you do some simple learning on your own. Just check the Wiki here on this board or read any number of other, readily available and free information on the web (including How to Brew, and you will find many answer to many questions.