Is there a way to create a highlevel task that will summarize all the subordinate tasks without indenting the subtasks?
This approach is not recommended because it will inflate the total work content
Let us assume that you can do what you are asking:
a way to create a highlevel task that will summarize all the subordinate tasks without indenting the subtasks
Then, in the Work Breakdown Structure (WBS) of the project, you will have the individual tasks counted once and the summary task counted again as a duplicate, thereby inflating the overall work content of the project. So, this is not recommended.
If you want a summary task in MS Project that includes all the project's tasks, you can simply set visible the "Project summary task". In the "Format" tab, check the "Project Summary Task" checkbox. The Project Summary Task always exists, but it is not shown by default.
If you want to summarize only a subset of tasks, then you will need to use custom fields with formulas. But as stated in Ashok's answer, this will duplicate some project variables (work, for example) so it's not a convenient option.