[Koha] Koha patron card number length

Hello, Folks - I am a Computer Tech for a public library in Michigan. We are looking at Koha as a possible replacement for our aging Dynix system. I have Koha running in a VMware virtual machine, running on Ubuntu Server 7.04. The install was not at all easy, but what do you want for free, eh? When I tried to add a user, the first thing Koha told me is that the card number was "Invalid cardnumber". We use a 14-digit codabar barcoded patron library card system. An example is "23330001535735". It is crucial that we be able to keep this intact, otherwise we will not be able to even consider Koha. Is there a way to modify this? Where is the documentation that talks about such things? Thanks very much, Mark