Just a quick question.

You need to check your local electrical code on this. Normally a spa needs its' own GFCI type breaker and be run from the main or a sub panel. Because of the mix of electricity and water, you need to comply with electrical codes. Done wrong, and someone could end up as toast floating in your spa. Not worth taking shortcuts on this type installation. Just my opinion.

If the line goes to a breaker panel, and you can appropriately add a breaker for the spa, "probably".

If you want to pig tail onto the existing wires, NO.

Check with a local licensed electrical contractor to be sure.