example

$ tree x
x
└── y/
    └── z/
        └── .gitignore

2 directories, 1 file

$ tar cf x.tar x

$ tar tvf x.tar
drwxr-xr-x  0 isaacs staff       0 Aug 17 16:47 x/
drwxr-xr-x  0 isaacs staff       0 Aug 17 16:46 x/y/
drwxr-xr-x  0 isaacs staff       0 Aug 17 16:46 x/y/z/
-rw-r--r--  0 isaacs staff       0 Aug 17 16:46 x/y/z/.gitignore

$ node rename.js

$ tree output/
output/
└── x/
    └── y/
        └── z/
            └── .npmignore

3 directories, 1 file

rename.js

'use strict'
const tar = require('./')
const file = 'x.tar'
const mkdirp = require('mkdirp')
const path = require('path')
const rimraf = require('rimraf')

rimraf.sync('output')
mkdirp.sync('output')
tar.x({
  file: file,
  cwd: 'output',
  onentry: entry => {
    if (path.basename(entry.path) === '.gitignore')
      entry.path = path.dirname(entry.path) + '/.npmignore'
  }
})