Can we add a new stage in .gitlab-ci other than the stages from the template file

Question

I have a template file which has few stages like:

template.yaml

stages:
    - a2
    - a3

Now, I have another file .gitlab-ci.yaml which should include stages from above template file, but also needs few more stages:

include:
  - project: 'path to repo'
    ref: main
    file: "/template.yml"

stages:
  - a1

job1:
   stage: a1

job2:
  stage: a2

I get the following error:

 chosen stage does not exist; available stages are .pre, a1, .post

Is it possible to do something similar without making any changes in the template file ?