Create new pallet items

Please forgive my technical ignorance, I'm not sure if I selected the proper items from dropdown lists. I could not find the question I want to ask. Please tell me if there is any way to add custom symbols to the pallets that are not already available in the Master Pallet. I am experimenting with alternative clefs, namely an "E" low bass clef centered on E2 for tuba. Anyone who plays tuba knows most of the notes you play will be a few ledger lines below the staff on a normal bass clef. While I know I could just use the 8va bassa, or 15va bassa bass clef, I'd like to be able to innovate if there are other symbols I'd like to invent. Can you tell me the following: 1. what is the file format used to communicate these glyphs to the app, and what graphic editor should I use to make them, and 2. What folder or directory should the glyph be saved to so that it can be found in the app? Thank you for your help.