AWS CloudFormation CodePipeline, ParameterOverrides, pass list to a nested stack

The scope of the question:

AWS CodeBuild, ParametersOverrides section

Using Parameter Override Functions with CodePipeline Pipelines

1) I can pass a string to ParameterOverrides, and then pass them to a nested stack, as it is described below

-create a string parameter

-pass it to ParameterOverrides section of the AWS CodeBuild project

-parse parameter in the nested stack

Quoting the official documentation: AWS::CloudFormation::Stack

If you use the Ref function to pass a parameter value to a nested stack, comma-delimited list parameters must be of type String. In other words, you cannot pass values that are of type CommaDelimitedList to nested stacks.

2) I can not figure out on how to transform a List to a string inside a ParameterOverrides section with the configuration below:

-define a Parameter with type of list ServiceSubnets: Type: List Description: Subnets associated with the service

-try to pass a parameter inside ParameterOverrides section as a value to a nested stack, apply Join function to transform it to a string

           ParameterOverrides: !Sub |
              {
                "ImageURI" : { "Fn::GetParam" : [ "BuildOutput", "imageDetail.json", "ImageURI" ] },
                "ApplicationRepoName": "${ApplicationRepoName}",
                "Cluster": "${Cluster}",
                "ListenerArn": "${ListenerArn}",
                "TargetGroup": "${TargetGroup}",
                "ServiceDesiredCount": "${ServiceDesiredCount}",
                "ServiceLoadBalancerPath": "${ServiceLoadBalancerPath}",
                "ServiceContainerPort": "${ServiceContainerPort}",
                "ServiceSecurityGroups": { "Fn::Join" : [ ",", "${ServiceSecurityGroups}"] ] },
                "ServiceSubnets": { "Fn::Join" : [ ",", "${ServiceSubnets}" ] },
                "TaskContainerPort": "${TaskContainerPort}",
                "TaskCpu": "${TaskCpu}",
                "TaskMemory": "${TaskMemory}",
                "TaskExecutionRoleArn": "${TaskExecutionRoleArn}"
              }

So I expect that the List should be transformed to a String and String should be passed and then used in the nested stack, however, attempt to deploy such stack returned an error:

Template error: variable ServiceSecurityGroups in Fn::Sub expression does not resolve to a string

My question:

Is it possible to use Join function inside ParameterOverrides section to transform a List to a String?

If yes, I would much appreciate if you share with me some example that illustrates how to do this.

Thank you.