Decode and then encode unknown class with NSKeyedArchiver?

Decode and then encode unknown class with NSKeyedArchiver?

I've got a project file from OS X application that is a plist generated
with NSKeyedArchiver. I need to programmatically change one string in it.
Basically, it contains NSDictionary object with Foundation classes. But
there is one custom class (GradientColor). I've defined it myself and
tried doing nothing in initWithCoder: and encodeWithCoder: but target
application crashes trying to read newly generated project file. So it
cannot handle nil values properly when initializing.
Can I somehow know which keys are corresponding to my class when
initializing it with initWithCoder:(NSCoder *)aDecoder in order to encode
them back unchanged?