You don't need wchar_t. The "extended" codes (c. 1985) are less than 255. For example, to draw the left lower corner of a double-lined box, use code 200 decimal, 310 octal ("\310") or 0xc8 ("\xc8").
Those characters need support from the terminal emulator you are using, but it should work fine.
edit
I have a vague memory of a 7-bit vs. 8-bit mode for old curses, but I cannot find any mention of it in the FSF ncurses 1.190 (2008/12/20), also identified as v5.7.3.20090207 which I have on Linux. The man page for curs_addch mentions symbolic constants for line drawing characters, so perhaps you are expected to use those instead of literal line drawing characters:
addch (ACS_ULCORNER); // upper left corner
for (int j = 0; j < boxwidth-2; ++j)
addch (ACS_HLINE);
addch (ACS_URCORNER); // upper right
...