I don’t think I fully understand the question (can you give an example?), but in fonts (and text display in general) you can only use glyphs that exist, so if you want to add a symbol that’s not part of the current Unicode standard, you can instead replace a symbol that is unlikely to be used with your font - there are many to choose from.
For right-to-left, don’t enter text in right-to-left mode in the “glyphs in the image” box, only in the font tester on the right.