At this time you can not but you can vote for the enhancement here: https://jira.atlassian.com/browse/CONFSERVER-24570
This app allows you to create sub-tasks https://marketplace.atlassian.com/apps/1222936/issuecreate-for-confluence?hosting=server&tab=overview
You must be a registered user to add a comment. If you've already registered, sign in. Otherwise, register and sign in.