Graphics card in Lenovo A520?

There's already an Intel HD graphics GPU in the processor, but I was wondering if a separate graphics card could fit in the Lenovo A520 All-in-one computer.

Nope. All in one computers are basically laptops and thus you can't really upgrade anything in them except maybe hard drive and RAM

Its not got a GPU - it's got a CPU and an Intel graphics driver.

No you can't fit a graphics card onto an AIO - it defeats the purpose of a AIO.