documentclass{article}
usepackage{expl3}

ExplSyntaxOn

seq_new:N l_mymod_seq
keys_define:nn { mymod } {
  a       .tl_set:N = tmpa, % some keys are treated special
  % others are grouped with unknowns
  b       .code:n = seq_put_right:Nn l_mymod_seq {(B~is~f{#1})},
  unknown .code:n = seq_put_right:Nn l_mymod_seq {(unknown~{l_keys_key_tl}~is~f{#1})},
}

newcommandsetkeys[1]{keys_set:nn { mymod } { #1 }}
newcommandprintkeys{seq_use:Nn l_mymod_seq {,~}}

ExplSyntaxOff

begin{document}
setkeys{a=5,b=6,c=7,d=8}
newcommandf[1]{textbf{#1}}
A is f{tmpa}
renewcommandf[1]{textit{#1}}
printkeys
end{document}