I'm not sure entirely what you mean by this, as WeBWorK already supports most AMS-TeX math macros, and uses the
amsart document class for its hardcopy output. Can you be more specific about what you are trying to accomplish?My understanding is that WeBWorK accepts TeX code only in math mode, i.e. inside \( . . . \) or \[ . . . \]. Is that correct?
Correct.
Is it possible to upload a file of one's favorite TeX macros?
You can include macros into a specific problem if you need them, but there is no mechanism for making macros available globally, and I would not recommend adding one, since that would mean that problems that you write would not work for others unless they also had your macros. It is best that you write problems that are as self-contained as possible.
The best alternative might be to write a macro file (in the PG sense) that defines the TeX macros that you want, and include the macro file with the problems that you write, but even that means that people will need to be sure to have your PG macro file available in order to use your problems.