Technical Term for the Sum I + Q
This can be implemented, without needing to access the real and imaginary parts of $z$ individually, by writing

$$\operatorname{Re}(z) + \operatorname{Im}(z) = \frac{(1 + i)(z^* - iz)}{2},$$

where $i$ is the imaginary unit and $z^*$ is complex conjugation.

Factoring out a complex constant, $\xi = 1/2 + i/2$, turns this into a slightly tidier expression,

$$\operatorname{Re}(z) + \operatorname{Im}(z) = \xi(z^* - iz).$$

These expression can be derived by applying the trigonometric identities

$$\cos(x) = \operatorname{Re}(e^{ix}) = \frac{{e^{ix} + e^{-ix}}}{2}$$
$$\sin(x) = \operatorname{Im}(e^{ix}) = \frac{{e^{ix} - e^{-ix}}}{2i}.$$

Perhaps this will give you some inspiration for choosing a good function name.