tar にファイルを追加するときに、どのように任意の先行パスを指定できますか?

GNU tar では、--transform (--xform ) は、ファイル名の変換を行うオプションです。ドキュメントと例はここにあります

BSD tar では、-s pattern 名前変換を行うためのオプションです。 freebsd.org ドキュメントからの引用:

-s  pattern
     Modify file or archive member names according to pattern.  The
     pattern has the format /old/new/[ghHprRsS] where old is a basic
     regular expression, new is the replacement string of the matched
     part, and the optional trailing letters modify how the replace-
     ment is handled.  If old is not matched, the pattern is skipped.
     Within new, ~ is substituted with the match, \1 to \9 with the
     content of the corresponding captured group.  The optional trail-
     ing g specifies that matching should continue after the matched
     part and stop on the first unmatched pattern.  The optional
     trailing s specifies that the pattern applies to the value of
     symbolic links.  The optional trailing p specifies that after a
     successful substitution the original path name and the new path
     name should be printed to standard error.  Optional trailing H,
     R, or S characters suppress substitutions for hardlink targets,
     regular filenames, or symlink targets, respectively.  Optional
     trailing h, r, or s characters enable substitutions for hardlink
     targets, regular filenames, or symlink targets, respectively.
     The default is hrs which applies substitutions to all names.  In
     particular, it is never necessary to specify h, r, or s.