How to convert hex escaped characters to unicode characters?

How to convert hex escaped characters to unicode characters?

I am revamping an old Emacs / Gnus configuration dating from before
unicode Emacs (21.3 or 22). In this configuration, I was using some
unicode characters to show usenet threads:
(setq gnus-sum-thread-tree-root "\x490a4 ") ; "> "
(setq gnus-sum-thread-tree-false-root "\x490a4 ") ; "> "
(setq gnus-sum-thread-tree-indent " ") ; " "
(setq gnus-sum-thread-tree-single-indent "") ; ""
(setq gnus-sum-thread-tree-leaf-with-other "\x4903c\x49020\x4904c\x490f9
") ; "+-> "
(setq gnus-sum-thread-tree-vertical "\x49022 ") ; "| "
(setq gnus-sum-thread-tree-single-leaf "\x490b0\x49020\x490f9 ")) ;
"\\-> "
But now with Emacs 23.2.1 I see empty squares instead of curved arrows. I
suspect it can be either a fontset or an escaping problem related to
unibyte / multibyte.
To exclude the escaping problem or solve it, how can I retrieve the
unicode characters to use the \u1234 escaping instead of the \x12345 ?
Thanks.