Tar Padding Nulls Advice

[Tom Kredo]

We have a data base program that allocates nulls in the middle of
the file to use for data expansion.  This may be created as a
1MB file, but only be a 1KB file of data.  UNIX only really allocates 1 block
for this file.  If we tar, cpio, or cp this file, the file becomes
fully allocated as the nulls are fully padded.  This is a problem
since a backup may no longer fit on the same file system.  Any
suggestions as to ways around this other than hacking tar, cpio, and cp?
Thanks!