We've got a Linux install with a few 'bi-' drives (accessible both to Windows and Linux), so I put this macro together :

%macro conv2url(fqfile);
%put &=fqfile;
%let cpp = &fqfile;
%let cpp2 = %sysfunc(dequote(&cpp));
%put &=cpp &=cpp2;

/* determine current path and file */
%let posit = %sysfunc(FINDC(%qleft(%superq(fqfile)), %str(\), b) );
%Let conv2urlcp = %Substr(%qleft(%superq(fqfile)),1,&posit );
%Put 0 &=conv2urlcp;
%let posit = %sysfunc(FINDC(%qleft(%superq(fqfile)), %str(\), b) );
%Let filename = %Substr(%qleft(%superq(fqfile)),&posit + 1,%length(&fqfile)-&posit );
%Put 1 &=conv2urlcp &=filename;
%let driveletter = %Substr(%qleft(%superq(conv2urlcp)),1,1);
%put &=driveletter;
%let url = &&drive&driveletter.%str(\);
%put &=url;
%let drive = %Substr(%qleft(%superq(conv2urlcp)),1,3);
%Let conv2urlcp = %SysFunc( TranWrd( %qleft(%superq(conv2urlcp)) ,&drive, &url ) );
%LET conv2urlcp= %SysFunc( TranWrd( %qleft(%superq(conv2urlcp)),%str(\),%str(/)));
%Put 2 &=conv2urlcp;
%put &=filename;
%let urlfile = &conv2urlcp.&filename;
%put &=urlfile;
&conv2urlcp.&filename;
%mend conv2url;

So with this global variable:
%let DRIVES = /mnt/nfs/prod/Teams/statutory/REGREP;

I can do this:
%let urlfile=%conv2url(S:\_Louisiana\173Report\Vendor_Files\Transportation\Report-1017.xlsx);
%put &=urlfile;

And get this:
URLFILE=/mnt/nfs/prod/Teams/statutory/REGREP/_Louisiana/173Report/Vendor_Files/Transportation/Report-1017.xlsx

And then I can do a proc import on that file, for example.

Or someone can enter the S: path and filename and the macro will convert it for them.