How to add custom symbols is explained in How can I use my own node symbols in yEd?
Re.:
is it present, in your development schedule, to insert the other flowchart symbols?
We currently do not plan to add additional flowchart symbols to yEd.
Is it not possible to export the Standard yED flochart palette extend it whit the missing graphic elements?
It is not possible to modify yEd's built-in palette sections. You can, however, create a user-defined palette secion as a copy of a built-in section and add additional elements to the copied section.