Add new symbol to FrameMaker 7.2's Symbol drop-down list

I need to add a new symbol to FrameMaker 7.2's Symbol drop-down list -- the "#" pound symbol.

I've never attempted to customize FrameMaker in this way. I tried reading the
customizing_frame_products.pdf file, but am not able to figure out how to add a new symbol.

If anyone would be kind enough to help me with easy to understand instructions for adding a new symbol, I would very much appreciate it.