How to add frames to symbols?

go to the symbols timeline (by double clicking the symbol) and right click over the layer the layer you want to extend > click one of the insert options

Hi.

You either need to double-click on a symbol instance on stage or in the Library to go inside the symbol and once inside of it you need to position the timeline's playhead where you want to add frames and then press F5 or F7.

This doc should give you more details:
https://helpx.adobe.com/animate/using/timeline.html

Regards,

JC