For Floating Energy and Power, can you stack up them, such as Kai'Sa Legends ability?

Yes, you can "stack up" (float) energy and power in your rune pool, including power generated by Kai'Sa's ability.

According to Rule 162.3, players may use a physical tracker to retain unspent Energy and Power over the course of their turn. Your pool of energy and power only empties at the end of the draw phase and at the end of the turn.

Regarding Kai'Sa specifically, here is how that interaction works based on the official rules and FAQs:

Key Mechanics

• Floating Resources: You can activate Kai'Sa's ability at any time you have priority (or as a reaction when paying costs) to add power to your rune pool. This power remains available in your pool until the end of the turn.
• No Chain for Adding: As noted in Rule 416.2 and the text on Kai'Sa's card, abilities that "Add" resources finalize immediately and do not use the chain. You cannot react to the act of adding resources itself.
• Usage Restriction: While you can accumulate power from multiple activations of Kai'Sa, that power is restricted by the ability's text: it can only be used to pay for spell costs (not for units, energy costs, or other abilities).

Sequence for "Stacking"

1. Activate: You exhaust Kai'Sa to activate her ability.
2. Add: 1 power is added to your rune pool. Because this is an "Add" ability, it finalizes immediately.
3. Repeat: You may activate her ability again (if you have other ways to ready her) or activate other "Add" abilities to continue increasing the total power in your pool.
4. Spend: When you play a spell, you can draw from this accumulated pool of power to pay the spell's power costs.

Important Nuance: As clarified in RiftJudge FAQ #8110, Kai'Sa's power can only be used to pay the colored power costs on spell cards, not the numbered generic costs that you normally recycle runes for. You still need to recycle the required number of runes for a spell's generic cost separately.