specific adding in a text file

[Chompy]

well, as the title says, I'm trying to change something inside a text file (.arc)

my arc files are like this:

arcnames=
folders=
amount=
account=

Now, all the weapons of the player is stored inside this arc file, into arcnames.
I usually have to do it manually, but, I haven't found a method to edit one member of the file, I made a thread about this, but didn't get what I really wanted (I got confused, in a other way), I got told I could use obj.add();
but how do I use it with loadvars and savelines and such?

I just want to add a string to arcnames, without rewriting the whole script with savelines.. so..

any thoughts?