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have you tried just


\usepackage{bm}
\bm{\nabla}


Yes, I have tried using the boldmath package. And it seems to give a "faked bold" \nabla which doesn't look very good.

That aside, my general question is how to selectively pick one math symbol font from one package and use it in another math ...

It appears that there isn't a bold version of the math symbol \nabla in the fouriernc package. Is it possible, though may or may not be preferred, to substitute this specific math symbol by selectively choosing it from another math font package, for example, the mathpazo or lmodern package. Will ...