I didn't take time to properly understand your code or to test this, but here's my idea of what clean code looks like:
import os, streams, strutils
var appendCodeBlock = ""
var filesWeAppendTo: seq[string]
let wildcard = getCurrentDir() / "*.nim"
for file in wildcard.walkFiles:
if file.getFileSize != 427: continue
let code = file.readFile
if code.contains("#marker"):
appendCodeBlock &= "\n\n\n" & code
else:
filesWeAppendTo.add file
for file in filesWeAppendTo:
let fs = newFileStream(file, fmAppend)
fs.write appendCodeBlock
fs.close