Calling non-\long macro as \long

You can not turn off the error check however plain (and latex) have \endgraf which is \let (rather than \def) to the primitive \par and is there for exactly this reason so you can use

\short{a\endgraf b}

the only other alternative would be to define a \long version of the command, which is trivial if copying the source is a possibility, but can be tricky if just the macro is available, especially if it has delimited arguments. If etex is available, basically you can use \scantokens to re-parse a \long\def that can be constructed from the output from \meaning\short.

If you have a simple macro, say with two undelimited arguments, it's possible to patch it for becoming \long without e-TeX:

\def\foo#1#2{#1-#2}

\begingroup\toks0=\expandafter{\foo{#1}{#2}}
\edef\x{\endgroup\long\def\noexpand\foo##1##2{\the\toks0 }}\x

\foo{a\par b}{c\par d}

{\tt\meaning\foo}

(see https://tex.stackexchange.com/a/39980/4427 for an explanation of the trick).

If the macro has delimited arguments or you don't want to bother knowing how many arguments it takes, \scantokens is your friend, but requires an e-TeX enabled engine, such as pdfTeX, XeTeX or LuaTeX.

\def\makelong#1{%
  \expandafter\getparts\meaning#1\relax
  \domakelong#1%
}

\begingroup
\catcode`M=12 \catcode`A=12 \catcode`C=12 \catcode`R=12 \catcode`O=12
\lowercase{\endgroup\def\getparts#1MACRO:#2->#3\relax}{%
  \def\prefixes{#1}%
  \def\parameters{#2}%
  \def\replacement{#3}%
}

\def\domakelong#1{%
  \edef\temp{\long\prefixes\def\noexpand#1\parameters{\replacement}}%
  \scantokens\expandafter{\temp}%
}

\protected\def\short#1{==#1==}
\def\nonlong#1+#2-{(#1)--(#2)}

\makelong\short
\makelong\nonlong

\short{a\par b}

\nonlong X\par Y+Z\par W-

{\tt\meaning\short}

{\tt\meaning\nonlong}

\bye

If you are using LuaTeX then you can set \suppresslongerror to a positive value. For example

\def\short#1{#1}

\suppresslongerror=1
\short{a

b}
\suppresslongerror=0

or

\long\def\forcelong#1{\suppresslongerror=1 #1\suppresslongerror=0 }

\forcelong{\short{a

b}}