DeclareSourcemap{
  maps{
    map{
      step[fieldset=entrykey, fieldvalue={<entryke>}]
      step[fieldset=note, fieldvalue={text for <entrykey>}]
    }
  }
}

begin{filecontents}{biblatex-dm.cfg}
DeclareDatamodelFields[type=field,datatype=literal]{notesonlyformystudents}
DeclareDatamodelEntryfields{notesonlyformystudents}
end{filecontents}

DeclareFieldFormat{notesonlyformystudents}{#1}
renewbibmacro{finentry}{finentryparprintfield{notesonlyformystudents}}

documentclass[a4paper,10pt]{article}
usepackage{natbib}
usepackage{datatool}
usepackage{etoolbox}

%% database for personal temporary notes
DTLnewdb{bibnotes}

%% command for notes
defbibnote#1#2{%
  DTLnewrow{bibnotes}
  DTLnewdbentry{bibnotes}{mylabel}{#1}
  DTLnewdbentry{bibnotes}{mynote}{#2}
}

%% patching the output
makeatletter
%% if natbib is loaded
patchcmd{@lbibitem}%
  {item[hfilNAT@anchor{#2}{NAT@num}]}%
  {%
    item[hfilNAT@anchor{#2}{NAT@num}]%
    DTLforeach[DTLiseq{mylabel}{#2}]{bibnotes}{mylabel=mylabel,mynote=mynote}{textit{mynote}}
  }{}{message{^^JPatching failed^^J}}%

%% if natbib is not loaded
% patchcmd{@lbibitem}%
%   {item[@biblabel {#1}hfill ]}%
%   {%
%     item[@biblabel {#1}hfill ]%
%     DTLforeach[DTLiseq{mylabel}{#2}]{bibnotes}{mylabel=mylabel,mynote=mynote}{textit{mynote}}
%   }{}{message{^^JPatching failed^^J}}%
% patchcmd{@bibitem}%
%   {item}%
%   {%
%     item%
%     DTLforeach[DTLiseq{mylabel}{#1}]{bibnotes}{mylabel=mylabel,mynote=mynote}{textit{mynote}}
%   }{}{message{^^JPatching failed^^J}}%
makeatother

begin{document}

cite{GhJaEn02,MeGa05,MuBeBoRo99}

bibnote{GhJaEn02}{This is important }
bibnote{GhJaEn02}{to understand chickens!}
bibnote{MeGa05}{This is about penguins:}
bibnote{MuBeBoRo99}{Cheese!}

bibliographystyle{plainnat}
bibliography{test}
end{document}

makeatletter
newcommand{bibnote}[2]{@namedef{#1note}{#2}}
makeatother

renewbibmacro{finentry}{finentryparcsname thefield{entrykey}noteendcsnamefinentry}

FUNCTION {fin.entry}
{ add.period$
  write$
  newline$
  "parcsname " write$
  cite$ write$
  "noteendcsname" write$
  newline$
}