How to assume an IAM role created from another stack and apply it to all your functions in Serverless Framework

I want to assume an IAM role that's already been created in another serverless.yml file. It seems as if using the iam property is the only (?) way to do this for all of the iam functions at once. The source code I've encountered mainly uses iamRoleStatements to apply IAM permissions, but that doesn't seem to be made to actually have the option to assume already created roles.

Secondary question, should I use the ARN of the role or create an export for it from the stack where it's being created?

provider:
  name: aws
  runtime: python3.8
  stage: ${opt:stage, 'dev'}
  region: ${opt:region, 'eu-west-1'}
  iam:
    role: arn:aws:iam::123456789012:role/execution-role
  iamRoleStatements:
    - Effect: Allow
      Action:
        - events:PutEvents
      Resource: arn:aws:events:${self:provider.region}:#{AWS::AccountId}:blablabla-${self:provider.stage}
    - Effect: Allow
      Action:
        - states:SendTaskSuccess
      Resource: arn:aws:states:${self:provider.region}:#{AWS::AccountId}:stateMachine:${self:provider.stage}-blablabla