How to change the duration options in classifieds


First let me say thanks for the previous support in getting the classifieds to appear.

So right now the classifieds gives a user the option of 1 week, 2 weeks, 4 weeks etc....

How do I change this? I have custom press installed - so I saw where I could go in and change what's displayed - but I suspect that only changes what appears - not what actually happens.

I saw another thread where someone asked and they were given advice to edit the plugin core files.

Is there anything that can be done? We want to have an option where users can go 6 months or 1 year for a classified listing.

I know there are plugins that allow you to insert code into your functions.php without ruining the core files - is there anything we can do that way ? If I understood code wise what's required I could deploy it that way.