No, it can only use the actions in its statblock
The text for the Steel Defender feature reads:
It can move and use its reaction on its own, but the only action it takes on its turn is the Dodge action, unless you take a bonus action on your turn to command it to take one of the actions in its stat block or the Dash, Disengage, or Help action.
The feature specifies that the action must be from its statblock.
The description of the Magic Stone spell reads:
You or someone else can make a ranged spell attack with one of the pebbles [...]
It allows someone other than the caster to use the stone, but it still requires you to take the Attack action to do so. The rule that allows someone else to use the stone is general because it refers to everyone who is not the caster. The rule regarding the Steel Defender is specific because it only applies to the Artificer's use of a feature as a bonus action. A general rule allowing other creatures to use the stone cannot overrule the specific rule defining the Artificer's use of a feature.