Simulink allows users to use annotations to label block diagrams and provide more information about its contents. Both text, and image annotations can be created. Please refer to the following documentation link for more information on adding annotations to models:
In this case, to add dummy arrows, you can follow the workflow used for adding ‘Image Annotations’ as mentioned in the above link. You will need images of uni-directional and bi-directional arrows, which you can add to the model as annotations.
I hope this helps.
