Marked solution is incorrect.
This is a recent feature added in TestComplete v14.50 as I was investigating the use of this. There is a full guide on this which can be found on the support site below.
https://support.smartbear.com/testcomplete/docs/testing-with/object-identification/name-mapping/xpath-expressions-and-css-selectors.html
Note - I have tried to implement mapping with CSS selectors alongside my existing name mapping which uses attributes and have had inconsistent mapping results.
For example I map a new object, which on the UI is within an existing mapped object and TestComplete maps it correctly as I would expect within the existing object at alias level, but not anywhere close to where I would expect at the mapped object level.
I contacted SmartBear about this, their suggestion? Delete all my existing mapping and start again if I want to work with CSS locators, hardly a solution, so just be aware of this if you do decide to go down the CSS locator route.
I would suggest if the above is something you would like to do, then up vote the feature request below which would allow us to transition our existing mappings over, rather than deleting and mapping all over again.
https://community.smartbear.com/t5/TestComplete-Feature-Requests/Request-adding-CSS-Selector-to-extended-properties-in-Object/idi-p/205255