Using added text characters

Instead of replacing characters like @ or so, I decided to go adding new ones on the "empty" addresses, but now I don't know how to "call" them from the dialog editor...

In the "Bitmap Editor"/"gfx" section of FF3usME, under the Big Font tab there is a table with the character graphics. The left column gives the hex "Index" of each character. To insert a character by hex value, add 0x20 to its "Index" value and enter it as a raw hex entry <$xx>.

So for example, the character "Z" has index 0x19; add 0x20 to get 0x39.
In the dialog editor, enter <$39> and click "Apply". The "Z" will appear in the caption.

Unfortunately, as @Madsiur mentioned, everything above <$7F> will enter a tuple from the DTE table.
So you are really limited to index 0x00 - 0x5F, and 0x5F is the space character.

...At least, if you are using FF3us as your base. FF3usME makes mention of the extended Kanji character banks used by FF6j -- not supported by FF3us.

It may be possible to work around this limit by dummying out the DTE table lookup in code, and then wiping out the DTE table itself so that the dialog editor saves the script character-for-character.
The script would then take up much more space, but FF3usME can expand the ROM to accommodate.

 I don't recall if there are similar restrictions for the small font, the one used for menus.

For the small font, you should generally avoid altering the following blank tiles:
00, 01, 02, 03, 04, 05, 06, 07, EE, EF, FE, FF