How do I open a file that is already in use?

Use DosOpen with OPEN_SHARE_DENYNONE.

/* this will copy an open program */

#define INCL_NOPM
#define INCL_DOS
#include <os2.h>
#include <stdio.h>
#include <string.h>

void usage(void)
{
    printf("USAGE: CopyOpen <source file> <dest file>\n\n");
    printf("       This program,  unlike the normal copy and xcopy commands,\n");
    printf("       will copy an open file.\n");
    printf("NOTE:  Wildcards are not supported\n");
}

int cdecl main(int argc, char **argv)
{
    HFILE hf, hfOut;
    USHORT usAction, rc, bytesRead, bytesWriten ;
    static BYTE buf[4096];
    long total=0l;
    int error = FALSE;

    if(argc!=3){
        usage();
        return 1;
    }
    rc = DosOpen(strupr(argv[1]),
        &hf,
        &usAction,
        0L,
        FILE_NORMAL,
        FILE_OPEN,
        OPEN_ACCESS_READONLY | OPEN_SHARE_DENYNONE,
        0L);
    if(rc){
        printf("SYS%04u: Could not open %s for read.\n", rc, argv[1]);
        return 3;
    }
    rc = DosOpen(strupr(argv[2]),
        &hfOut,
        &usAction,
        0L,
        FILE_NORMAL,
        FILE_CREATE | FILE_TRUNCATE,
        OPEN_ACCESS_WRITEONLY | OPEN_SHARE_DENYREADWRITE,
        0L);
    if(rc){
        printf("SYS%04u: Could not open %s for write.\n", rc, argv[2]);
        return 3;
    }
    else{
        do{
            rc = DosRead(hf, buf, sizeof(buf), &bytesRead);
            if(!rc){
               rc = DosWrite(hfOut, buf, bytesRead, &bytesWriten);
               if(!rc) total += bytesWriten;
            }
        }while(!rc &&
               sizeof(buf) == bytesRead &&
               bytesRead == bytesWriten);
        if(rc){
            printf("SYS%04u: while copying.\n", rc);
            error = TRUE;
        }
        if(bytesRead != bytesWriten){
            printf("Disk full?\n");
            error = TRUE;
        }
        DosClose(hf);
        if(!error)
            printf("%lu bytes copied.\n", total);
    }
    return( error? 3 : 0);
}

Credit: Peter Fitzsimmons