\def\prepend#1#2{% #1=string  #2=macro containing a string
	\def\tempstring{#1}%
	\expandafter\expandafter\expandafter
	\def\expandafter\expandafter\expandafter
	#2\expandafter\expandafter\expandafter
	{\expandafter\tempstring#2}%
}
\def\mystring{world!}
\prepend{Hello }\mystring
Result : \mystring
\bye