Sorry, it isn’t available. The preinstalled watch faces have access to core level functionality that is not available to Galaxy Watch Studio or even developers using Tizen Studio.
Sorry for replying to such an old post, but I wanted to help out anyone else googling this specific thing
If you add a Complication in Watch Face Studio and set it to Editable, you can set the complication to anything your watch supports from the watch itself. So I was able to add a timer complication by adding an empty complication in Watch Face Studio, then loading it on the watch, hold-pressing and clicking the edit button, then setting the complication to Timer.
Please note that this topic was about Galaxy Watch Studio, watch face editor for older watches with OS Tizen. Those watches wont get any kind of update to become compatible with Watchface studio and its complications.