Let’s say Bob is a second level sorcerer attuned to a Ring of spell storing. His buddy Fred casts some spells inside of his Ring. But greedy Bob wishes to convert the energy of these stored spells into sorcery points (as a bonus action, as per the Font of Magic feature). Is Bob allowed to do so ?
Converting a Spell Slot to Sorcery Points. As a Bonus Action on your turn, you can expend one spell slot and gain a number of sorcery points equal to the slot's level.
Casting the spell from the ring of spell storing or simply converting that spell is not expending one spell slot, it's casting a spell (or using the ring).
More specifically, the Ring of Spell Storing stores spells, not spell slots, although it is limited by the spell slots.
This ring stores spells cast into it, holding them until the attuned wearer uses them.