How to transform lambda function to multi-argument lambda function and how to rewrite or approximate terms?
You are looking for currying and uncurrying which transform functions of type $A \times B \to C$ to functions of type $A \to (B \to C)$, and vice versa. Currying takes IS to is, while uncurrying takes is to IS. This is a standard and very basic technique in $\lambda$-calculus. There are many manifestations of currying and uncurrying, for instance in arithmetic it is the identity $c^{a \cdot b} = (c ^ b) ^ a$.

It is largely a matter of taste how you write multi-argument functions. The uncurried form $A \to (B \to C$ has the advantage that you can apply the function only to the first argument. In your case is boss is a useful concept which is expressible very neatly. With the other function you have to write λ x . IS (boss, x).

Note however that sometimes people do not include product types $A \times B$ in their versions of $\lambda$-calculus, in which case of course currying is unavailable (but nothing much is lost).