You can’t. Dynamic numeric icons only support values 0 to 100 and the “everything else” default option.
Bit more detail
Potentially, you could copy your negative number to a positive “shadow” Item, and use sitemap visibility= feature to switch display lines between “real” and “shadow” Item - with a different set of icons - switching based on negative or positive value of “real” Item https://www.openhab.org/docs/configuration/sitemaps.html#visibility
