\documentclass{article}
\RequirePackage{filecontents}
\begin{filecontents*}{myfonts.sty}
\RequirePackage{expl3,l3keys2e}
\ProvidesExplPackage {myfonts} {2012/03/05} {1}{}
\keys_define:nn { myfonts }
{
font .code:n = \cs_set_eq:Nc \myfont { #1 },
font .default:n = sffamily,
}
\keys_set:nn { myfonts } { font }
\keys_define:nn { myfonts }
{
unknown .code:n =
{
\msg_warning:nnx { myfonts } { unknown-option }
{ \exp_not:V \l_keys_key_tl }