It is easily possible to add arbitrary field contents to entries with Biber's sourcemap features.
With
\documentclass[british]{article}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{babel}
\usepackage{csquotes}
\usepackage[style=authoryear, backend=biber]{biblatex}
\DeclareSourcemap{
\maps[datatype=bibtex,overwrite=false]{
\map{
\step[fieldsource=entrykey, match={.*}, final]
\step[fieldset=file, fieldvalue={$1.pdf}]
}
}
}
% just to print the 'file' field
\DeclareFieldFormat{file}{file: \path{#1}}
\renewbibmacro{finentry}{\printfield{file}\finentry}
\addbibresource{biblatex-examples.bib}
\begin{document}
\cite{sigfridsson,nussbaum,worman,geer}
\printbibliography
\end{document}

You would add to each entry the equivalent of
file = {<entrykey>.pdf},
to all entries that don't already have a file
field.
You don't even need RegExes here, you could just say
\DeclareSourcemap{
\maps[datatype=bibtex,overwrite=true]{
\map{
\step[fieldsource=entrykey, final]
\step[fieldset=file, origfieldval, final]
\step[fieldset=file, fieldvalue={.pdf}, append]
}
}
}
which would overwrite an existing file
field. The overwrite=true
is needed because an append
step must formally be able to overwrite a field. You can guard against overwriting a pre-existing file
field by adding
\step[notfield=file, final]
as first step.
But it seems rather pointless to add a field file
with the sole contents <entrykey>.pdf
to all entries. The information you are adding is a very simple function of information that is already present in the entrykey
field, after all.
\documentclass[british]{article}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{babel}
\usepackage{csquotes}
\usepackage[style=authoryear, backend=biber]{biblatex}
\DeclareFieldFormat{fakefile}{file: \path{#1.pdf}}
\renewbibmacro{finentry}{\printfield[fakefile]{entrykey}\finentry}
\addbibresource{biblatex-examples.bib}
\begin{document}
\cite{sigfridsson,nussbaum,worman,geer}
\printbibliography
\end{document}
Would produce the same result but without the need to map any field contents.