Is there a way to merge or subtype type references in AsyncAPI

I have an AsyncAPI document that defines a message with an existing payload type.

channels:
  onboarding-consumption:
    publish:
      operationId: publishConsumptionEvent
      summary: Notify of application usage for consumption reporting purposes.
      message:
        name: consumptionEvent
        title: Consumption Event
        headers:
          $ref: "#/components/schemas/message-header"
        correlationId:
          $ref: "#/components/correlationIds/defaultCorrelationId"
        payload:
          $ref: "#/components/schemas/consumption"
        tags:
          - name: onboarding

Effectively I want to use the same existing payload type, but I want to add additional properties to that payload type, but in a different message definition. Is there a way to subtype (and potentially override) a schema or to merge schemas? The same question would apply to header types as well.

it is more related to JSON Schema. JSON Schema is one of many different formats you can use in AsyncAPI to describe your message payload definition.

In your case I believe you need allOf JSON Schema feature. More details in official docs

Below example, I extracted from this article.

subscriptionStatusCommon is the part that is the same in subscriptionStatusError and subscriptionStatusSuccess.

schemas:
    subscriptionStatusError:
      allOf:
        - properties:
            errorMessage:
              type: string
          required:
            - errorMessage
        - $ref: '#/components/schemas/subscriptionStatusCommon'
    subscriptionStatusSuccess:
      allOf:
        - properties:
            channelID:
              type: integer
              description: ChannelID on successful subscription, applicable to public messages only.
            channelName:
              type: string
              description: Channel Name on successful subscription. For payloads 'ohlc' and 'book', respective interval or depth will be added as suffix.
          required:
            - channelID
            - channelName
        - $ref: '#/components/schemas/subscriptionStatusCommon'
    subscriptionStatusCommon:
      type: object
      required:
         - event
      properties:
        event:
          type: string
          const: subscriptionStatus
        reqid:
          $ref: '#/components/schemas/reqid'
        pair:
          $ref: '#/components/schemas/pair'
        status:
          $ref: '#/components/schemas/status'
        subscription:
          required:
            - name
          type: object
          properties:
            depth:
              $ref: '#/components/schemas/depth'
            interval:
              $ref: '#/components/schemas/interval'
            maxratecount:
              $ref: '#/components/schemas/maxratecount'
            name:
              $ref: '#/components/schemas/name'
            token:
              $ref: '#/components/schemas/token'

I hope that helps.