Basically, you can create an array containing the symbol position for each trading session. This array is similar to other data arrays (the high/low/open/close ....) in the FinanceChart.
If there is no symbol for a certain trading session, you can use Chart.NoValue as the symbol position. You can then use addScatterLayer to that the scatter layer. As no x-coordinates are needed, the x-coordinates in addScatterLayer should be set to Nothing.