Can I file a patent for a software that includes functionality from existing patents?

Yes. The fact that someone has patented a concept that you are building upon does not stop you from getting a patent on an improvement on that concept. that is kind of the point of the patent system encourage people to disclose their invention (in exchange for possible patent protection), so that others will improve in the inventions that were disclosed.

However, having a patent on an improvement on someone else’s invention does not protect you from infringing on their patent. It may be that to use the invention of your patent, the user needs a licence from both you and the holder of the patent that you improved upon. Even though you may be able to get a patent on your invention, you would need to compare the claims of the patent that you are building upon to see if your invention is covered by the claims of that patent to determine if your invention infringes that patent. If you are not using at least one element that that patent’s claims require, it may be that your invention does not infringe that earlier patent, whereas if your invention includes each element of any one of the broadest claims of the patent that you improved upon, it is likely that anyone using your invention infringes on that earlier patent. Either way, your invention may still have value if your invention is one that people are willing to pay the extra royalties for the extra improvement (unfortunately however, most inventions never make money).

Source: Quora.com