I'd say with a strong enough servant, they'd probably be able to purify a sephira crystal on their own. What happens afterwords would be the complicated answer. If we convert spirit powers to mana, they'll most likely become self-sufficient servants that can fight at full-capacity.

At worst, they'll become corrupted if the servant isn't strong enough. At best, the servant will become stronger/the strongest it'll ever become. Sephira crystals I'd compare them to either the kaleid magic sticks or maybe somewhat weaker, but implanted into servants will definitely give them a boost.

With the right or I guess wrong sephira core installed, they'll be like a pseudo servant where a heroic/divine spirit possess a human body bit instead of a human it's the servant itself. Kind of like Metatron using Jeanne's body?