When adding an "Other assets", only limited currencies are available.
For currencies to be featured, there needs to be a reliable, trustworthy IOU issuer for the given currency.
You can easily add currencies by tapping the Add asset button on the home screen. This is a curated list with trustworthy assets. You can add any other asset using a tool like xrptoolkit.com / xumm.community and sign with XUMM.