Register existing template

[Alteczen]

I've seen multiple example on how to register a custom template in an existing template but not the other way around. If I create a custom template called 'mytemplate' can anyone provide me a working example to register a stock template so that it can render inside my custom template?

I'm trying to render the memberinfo_block_friends template inside my custom template. I've tried numerous hooks and have been beating my head against the desk for the past 3 hours.

Thanks in advance.