That I don’t know, I don’t think there is an option on the website or in the offline tool for that. Typically, you would want the minimum number of characters converted to reduce size of the font file, given MCU storage is often limited, and I think the tool is designed around that principle.
Maybe do a quick search online to see if there are any tools that can do a glyph/symbol dump from a ttf, and use that dump as your symbols in the tool?
I now use a script to call the offline version, and have the symbols I used saved in that script. If my code needs a new symbol, I just add the symbol to my script and run it again to regenerate the font files, which makes it significantly less painful than using the web tool to regenerate several font files, cutting and pasting symbols and ranges, etc. for each one…