Adding article-specific LaTeX macros

[tOgg1]

For some articles it would be great to have the possibility of creating local LaTeX macros. E.g. in the article TFY4165 right now there is a need for the inexact-differential symbol, which "needs" a macro to be defined, something along the lines of

\newcommand{\dbar}{\ensuremath{\mathchar'26\mkern-12mu d}}

(the example just to convince you this actually has use cases)

Also, could it be possible to import, in any way, a non-default package?