You can read a file into a buffer like:

        fsize&=GetFileSize&:(file$)
        REM GetFileSize is in sysram1.opx or systinfo.opx, not sure
        pBuffAddr&=ALLOC(fsize&)  REM alloc a buffer to hold the file
        ret%=IOOPEN(FileHandle%,file$,0)
        IF ret%<0
                RETURN ret%  REM if something goes wrong
        ENDIF
        
        REM read your file into buffer
        ret%=IOREAD(FileHandle%,pBuffAddr&,fsize&)
        IF ret%<0              
                IOCLOSE(FileHandle%)
                RETURN ret% REM if something goes wrong
        ENDIF
        IOCLOSE(FileHandle%)

Now you may be able to use the code I listed before, except that you 
have to change the size long:

        len&=fsize&  REM Instead of len&=LEN(data$)

and write the buffer like:

        IOWRITE(id%,pBuffAddr&,len&)
        REM instead of IOWRITE(id%,ADDR(data$)+1,len&)

And remember to free the buffer when you have finished:

        FREEALLOC(pBuffAddr&)
        
I can't remember if you have to change the footer, but it is worth 
trying out this code first.

Roger