YES!!! Having a box either above or below the "Exclude Symbols" box called "Include Only These Symbols" ( or similar wording) would be excellent! Just yesterday I signed up for a website which created the same problem as the one rmmech experienced.
Having Enpass to keep that box populated with the list of manually entered symbols on a site-specific basis (not showing up for other passwords saved in Enpass) would be great also.
You really do have a great password manager. Thank you!