NZFF

Can I do this in FSX? If so what is the easiest way? Will I need to learn how to use gmax?

What do you mean exactly? adding a default texture to a runway or adding a custom one?
Use FSXPlanner for the former...the latter is possible but Robin would need to help you with that

I want to add custom texures. Would I need a payware program?

I'm still not entirely sure what you want to do, but I'll take a stab.
It is reasonably easy to customise your own runway textures, but this is a global change -- that is, you can change the asphalt runway textures, for instance, but this will change every asphalt runway in the world.
I suspect, though, that you want to change one specific runway and one airport? This is trickier. The only way I know uses GMAX and the FS2002 gamepack.

Ok thanks for that. I think I'll pass as it seems too tricky for me