It doesn't insert it but tt can display a tbl file with Japanese characters in it.
As KingMike said, you will need to create a table file in Shift-JIS format first. You can also use a program like JWPce and create a .sjs file in there. WindHex can then load up the .sjs file and display the characters inside of it.
As an example, create a .sjs file in JWPce. Load your project into WindHex, choose File followed by Open Table File, and then select either Table #1 or Table #2. Find the .sjs file created in JWPce and select Open. The last thing you will need to do is to click the Option menu and choose "View Text Data as Unicode" (or just push Ctrl+D).
You should now be able to se the Japanese characters in WindHex.