Are you wishing to add more symbols to Rapidsketch?
The stock Rapidsketch Symbol library has many options contained within, but there are many things that are not represented in this library, or may not look the way you'd like them to. Rapidsketch allows you to add new symbols to the library, custom to you.
You can create new symbols by clicking, under the Symbols tab on the bottom right, the New Symbol button:
From there, a new window will pop up in which you can draw a new symbol. You must choose a name and a category under which this symbol will reside, and you can use any of the provided drawing tools to make this new symbol:
You can delete the measurements if desired, or you can leave them in to denote the size of the actual thing you're representing with this symbol.
Then, it'll appear as an items in your list when you save it:
And you can click and drag it into the sketch image: