Is it possible to set a small project, that would be paid for separately, as a task within another larger project?

If working with multiple suppliers / customers to deliver different outputs that will be combined at a later date, it can be useful to combine all the separate projects into one 'master project'. 

For example:

Project A - Make a cup of tea

Tasks

- Fill kettle

- Put kettle on

- Get teabag

- etc

Project B - Get Milk

- Go to shop

- Buy Milk

- etc

Can we set 'Project B - Get Milk' as a task within Project A?

I hope that makes sense!