No, unfortunately not. The task to which the subtasks get added has to be the target of the rule, which means it has to be in a project where the rule is applied.
One alternate approach I can think of:
Instead of using “Convert task to project”, add a rule to the “New Enquiries” project - the trigger would be adding a tag or setting a custom field to some value, and the action would be “Create a new project from a template”.
That way, when it was appropriate to turn an enquiry into a project, you would just add the tag or set the custom field. Flowsana would create the new project for you automatically (you can have the name of the enquiry task as all or part of the project name), from a template that would consist of those common tasks. (You could even have multiple “Create project rules”, each creating from a different template, if you wanted to have multiple sets of template tasks.)
Additionally, if you add Flowsana rules to the template, then those rules get duplicated into the new project. So on top of all of the above, you could also have “Create subtask” rules be part of the projects. (You might not even need “Create subtask” rules using the above approach but you could!)