portsnap, excluding parts of the ports tree

[F. Even - fbsd-questions]

I know I can run "portsnap extract sysutils/portupgrade" ...or something 
to that effect....but that will not "register" say for a "portsnap 
update" after a new "portsnap fetch".  This is what I will get for an 
error if I try that:

[root at gv:/usr/ports]$ portsnap update
/usr/ports was not created by portsnap.
You must run 'portsnap extract' before running 'portsnap update'.

...is there any way I can only maintain a small portion of the ports 
tree while still using portsnap (or should I not bother and just use 
cvsup)?  I'd like to keep the parts of the ports collection around that 
I use, but not all of it...as this box has space issues.

Thanks,
Frank