you could use CVS to track NetBSD as third-party source - from the CVS
info page

     Tracking third-party sources
****************************
     -----------------------------
   
           If you modify a program to better fit your site, you probably want
        to include your modifications when the next release of the program
        arrives.  CVS can help you with this task.
        
           In the terminology used in CVS, the supplier of the program is
        called a "vendor".  The unmodified distribution from the vendor is
        checked in on its own branch, the "vendor branch".  CVS reserves branch
        1.1.1 for this use.
        
           When you modify the source and commit it, your revision will end up
        on the main trunk.  When a new release is made by the vendor, you
        commit it on the vendor branch and copy the modifications onto the main
        trunk.
        

Bruce Martin writes:
 > We are currently using NetBSD/i386 very happily. We use a customised kernel,
 > because we have a number of PCI/ISA cards produced in-house, and so we write
 > device drivers for them for NetBSD. So, at the moment, we take a generic
 > /usr/src/sys tree, add a few of our own files, and then add a few lines to
 > conf.h, conf.c, files.pci, and those sorts of files so that the system will
 > "see" the new devices when compiling a kernel.
 > 
 > So, we can make these changes under, let's say 1.5, and create .patch files
 > for all files we modified. This makes it quite easy for any other developers
 > here to modify their generic /sys tree in the same way by applying the
 > .patches. All good and well.
 > 
 > The problem now arises: NetBSD 1.5.1 comes out, with all sorts of new
 > features, bugfixes, bells and whistles :) So now we want to upgrade to it,
 > and include all our own drivers and customizations. I obviously can't use
 > the .patch file anymore, because it changes from 1.5 to our customized, and
 > will probably get way confused if the source has changed between 1.5 and
 > 1.5.1.
 > 
 > Do we have to make all the changes in each file manually again, and generate
 > new .patch files? Is there any way to perhaps use the previous .patch files
 > (say, in conjunction with a 1.5->1.5.1 .patch) to get a 1.5.1 version, with
 > our customizations?
 > 
 > This used to be a simple process, as we only had to change 10 or so files,
 > but we are looking more like 100 files to change now, so it becomes very
 > non-trivial!
 > 
 > Thanks
 >  Bruce
 > 

-- 
Rex McMaster