Walnut oil over mineral oil?

JMHO you will end up with a gummy mess!

Problem with mixing mineral oil (non drying oil) and walnut oil (drying oil) will not give you any more sheen.

If can find a pure carnauba wax stick melt a little apply and buff it out might add little more sheen.

Mahoney sell an oil/wax combo not sure if he sells just wax. Some people like mixing bees wax and carnauba wax making easy to apply and because adds a little sheen over walnut oil.

You could use hot towels to remove the mineral oil. I would wait till bowl looks like needs another application of oil before trying anything else.

-- Bill