This will work for some number of files. You want to include "-print0" and "xargs -0" in case any of the paths have spaces in them. This example looks for files modified in the last 7 days. To find those modified before the last 7 days, use "+7".
find . -mtime -7 -print0 | xargs -0 tar -cjf /foo/archive.tar.bz2
As this page warns, xargs can cause the tar command to be executed multiple times if there are a lot of arguments, and the "-c" flag could cause problems. In that case, you would want this:
find . -mtime -7 -print0 | xargs -0 tar -rf /foo/archive.tar
You can't update a zipped tar archive with tar, so you would have to bzip2 or gzip it in a second step.