Yessir. Yessir it is.

But it's often not a simple copy-and-paste matter.

The Newbie Package of REQUIRED Material
If you're a rank beginner, read some of this stuff first. (And apologies for the "RTFM" part; that's not mine.)
ROMHacking.net FAQ: You ask, we answer!
ROMHacking.net Getting Started Section: Newbies Go HERE!
ROMHacking.net Documents Section!
How to ask questions the smart way.
On the Essence of ROM Hacking

Try some simple graphics hacks, then move onto the harder stuff.

Thanks.