Puppet users - Nov 2014 - Create resource if it doesn't exist, but do not update it if it does already exist

I've been poring through documentation and haven't found anything yet,
so
I'll throw the question out here.

With puppet, is there a way to define a resource in such a way that when 
applied to a client for the first time it is created, but if the resource 
already exists on the client nothing is changed?  Specifically, I'd like to 
do this with users.

Here is my situation: in the past my organization has poorly managed user 
UIDs across machines (multiple users in puppet with the same UID, creating 
users in puppet that conflict with system users' UIDs, etc).  This resulted 
in numerous, frustrating UID conflicts.  I've been working on cleaning this 
up, but the mass renumbering of important existing users has all sorts of 
production-impacting risks.  What I'd like to do is tell puppet to create 
these users with their new, conflict-free UIDs on new machines but have it 
not touch users that are already created on existing machines.  This will 
allow us to use a slow, methodical approach to fixing the UIDs on old 
machines to help mitigate the risks.

Any thoughts are appreciated.

Kris

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