Can I play Signature Spells of a champion not in my deck?

No, you cannot include or play signature spells for a Champion unless that Champion is your chosen Champion Legend.

According to RiftJudge FAQ #7473, "Signature Spells can only be included in your deck if your Legend matches. Having the Champion unit alone is not sufficient."

Furthermore, Rule 103.2.d.2 states: "All of the Signature cards must have the Champion tag that corresponds to the Champion Legend of the deck."

In summary:

• Your Champion Legend dictates your deck's identity and which signature cards you are allowed to include.
• You must have a Champion Legend that shares the same character tag as the signature spell you wish to include.
• Simply having a Champion unit in your deck that matches the signature spell's tag is not enough if that unit is not your chosen Champion Legend.