How to create IAM user with access key and secret key using Serverless framework?

you can do that by writing raw CloudFormation in resources section: https://www.serverless.com/framework/docs/providers/aws/guide/resources.

However, one note - it is not recommended to pass AWS Access Keys directly to Lambda - it's better to use IAM Role that is assigned to Lambda to grant it proper permissions.

EDIT: Example on how to access it

provider:
  name: aws
  environment:
    ENVVAR:
      'Fn::GetAtt':
        - MyKey
        - SecretAccessKey

resources:
  Resources:
    MyKey:
      Type: AWS::IAM::AccessKey
      Properties: 
        Serial: 0
        Status: Active
        UserName: username

I want to reiterate once again that it's a really bad idea to do something like the above - you should rely on temporary credentials from assumed role instead.

Here's how to add a user with roles and permissions set up. Setting up roles and permissions is important since usually you don't create a user with Access/Secret key in a vacuum, the user needs to have some accesses.

Notes:

• All IAM resources need to have a region in the name since redeployment a resource with the same name will cause an "unrecoverable error" (accordingly to AWS IAM documentation)
• To reference the Access Key you just specify resource name, but to reference the Secret Key you need to get property SecretAccessKey.
• Sample permissions in the policy give access to an SQS queue, but it can be anything

File serverless-with-my-user.yml:

resources:
  Resources:
    # Naming an IAM resource can cause an unrecoverable error if you reuse the same template in multiple Regions
    MyUser:
      Type: AWS::IAM::User
      Properties:
        UserName: "my-${self:provider.stage}-${self:provider.region}-User"
        Policies:
        - PolicyName: 
            Fn::Join:
              - ''
              - - "myUser"
                - '-'
                - ${self:provider.stage}
                - '-'
                - ${self:provider.region}
                - '-MyPolicy'
          PolicyDocument:
            Version: '2012-10-17'
            Statement:
            - Effect: Allow
              Action:
                - "sqs:GetQueueAttributes"
                - "sqs:GetQueueUrl"
              Resource: { 'Fn::ImportValue': '${self:provider.stage}-MyQueueArn' }
    MyUserKey:
      Type: AWS::IAM::AccessKey
      Properties: 
        UserName:
          Ref: MyUser

  Outputs:
    MyUserAccessKeyID:
      Description: The Access Key of the MyUser
      Value:
        Ref: MyUserKey
      Export:
        Name: ${self:provider.stage}-MyUserAccessKeyID
    MyUserAccessSecretKey:
      Description: The ARN of the secret key
      Value:
        Fn::GetAtt:
          - MyUserKey
          - SecretAccessKey
      Export:
        Name: ${self:provider.stage}-MyUserAccessSecretKey

And here's how to access this user's access key and secret key in other parts of serverless resources (e.g. lambda):

File serverless-with-my-service.yml

environment:
  MYUSER_AWS_ACCESS_KEY_ID:
    Fn::ImportValue: '${self:provider.stage}-MyUserAccessKeyID'
  MYUSER_AWS_SECRET_ACCESS_KEY:
    Fn::ImportValue: '${self:provider.stage}-MyUserAccessSecretKey'

If user, key and service are in a single file, there's no need to use output variables and you can reference the values directly.