結論: 素直に NodePath の addComment 使え

add-comment.js

const template = require('babel-template')
const {parse} = require('babylon')
const {transform} = require('@babel/core')
const prettier = require('prettier')

const assert = require('assert')

const src =
`function hoge(a: number): number {
  return a
}

function fuga(a: string): string {
  return a
}
`

const addLeadingComment = (nodePath, comment) => {
  // const temp = `${comment}\n${nodePath.getSource()}`
  // const ast = template(temp, {preserveComments: true, plugins: ['flow']})()
  // nodePath.replaceWith(ast)

  nodePath.addComment('leading', comment)
}

const addJsDoc = (nodePath, text) => {
  const comments = ['*', ...text.split('\n').map(line => ` * ${line}`), ' ']
  return addLeadingComment(nodePath, comments.join('\n'))
}

const plugin = (babel) => {
  const toParam = nodePath => {
    assert(nodePath.type === 'Identifier')
    let typeAnnotation = ''
    if (nodePath.node.typeAnnotation) {
      typeAnnotation = `{${nodePath.get('typeAnnotation.typeAnnotation').getSource()}} `
    }
    return `@param ${typeAnnotation}${nodePath.node.name} - parameter a`
  }

  const visitor = {
    Function: {
      exit: (nodePath, state) => {
        console.log(nodePath.getStatementParent().node.leadingComments)
        if (nodePath.getStatementParent().node.leadingComments) {
          return
        }
        const comments = []

        nodePath.get('params').forEach(param => {
          switch (param.type) {
            case 'AssignmentPattern': {
              comments.push(toParam(param.get('left')))
              break
            }
            case 'Identifier': {
              comments.push(toParam(param))
              break
            }
            default: {
              throw new Error(param)
            }

          }
        })

        if (nodePath.node.returnType) {
          comments.push(`@return {${nodePath.get('returnType.typeAnnotation').getSource()}}`)
        }

        addJsDoc(nodePath.getStatementParent(), comments.join('\n'))
      }
    }
  }
  return {
    inherits: require('@babel/plugin-syntax-flow').default,
    visitor
  }
}


let {code} = transform(src, {plugins: [plugin]})
// code = prettier.format(code)
console.log(code)