Look in the file kurier.sty and you'll find the lines (not in the same places)
\DeclareOption{regular}{\renewcommand{\rmdefault}{kurier}}
\SetMathAlphabet{\mathsf}{kurier}{OT1}{\rmdefault}{m}{n}
\SetMathAlphabet\mathsf{kurierbold}{OT1}{\rmdefault}{bx}{n}
that tell you how to do in your case. The first line reveals the font family internal name, the other two what's used for \mathsf. So, add
\DeclareMathAlphabet{\mathsf}{OT1}{kurier}{m}{n}
\SetMathAlphabet{\mathsf}{bold}{OT1}{kurier}{bx}{n}
to your document preamble. The first declaration tells LaTeX that \mathsf must use Kurier, changing it into bold when the math version is bold.
unicode-mathin xelatex/lualatex is very easy: https://tex.stackexchange.com/questions/30049/how-to-select-math-font-in-document – Cicada Sep 28 '19 at 22:24