The only thing that 'kit' would contain is the nib assembly as that is the only thing different between the two pens. Interestingly, or not, the wholesaler of that pen kit offers the rollerball nib, but not the fountain pen assembly for separate purchase. That's not to say that the fountain pen assembly isn't available as this supplier will provide any individual pen part with two requirements: 1) a minimum amount must be ordered and 2) they will submit the order when they make their next order for that kit. This means that you might have to wait some time before your order is filled.
What many of us do is simply keep a few extra of these pen kits on hand so that if a customer likes a pen that is set up as a rollerball but would prefer a fountain (or vice versa), we can simply swap out the bits and make the sale.
