Regras para criar um novo feitiço aparecem em Street Magic, pp. 158-162.
Alguns trechos:
A shamanic formula may be an item such as a painting, a carving, a set of runes, or a medicine bag. Hermetic formulae are typically represented as written/digital texts or complex diagrams composed of many arcance symbols. The formulae contain the same concepts but express them in different ways.... Any character who possesses the Arcana skill (see p.24) can design a spell formula.... Spell design is handled as an Arcana + Logic Extended Test. The threshold and interval are determined by the spell category, as noted on the Spell Design Table. (SM 158)
Um feitiço de Combate tem um Limite de 12 e um Intervalo de 3 meses. Existem vários modificadores condicionais na tabela que podem ser aplicados.
Se você copiar as estatísticas de feitiços existentes, acho que o custo deve ser o mesmo. Você vai querer usar o efeito elementar 'Ice' (p. 164):
Attacks with the Ice effect cover the target with a slick coating of frozen water. Treat the Ice effect as Cold damage (p. 154, SR4), except that objects or terrain affected will be encrusted with ice. Anyone trying to cross an icy surface may need to succeed in an Agility + Reaction Test to avoid slipping (with a threshold equal to the attack's net hits); vehicles must make a Vehicle Test (p. 159, SR4). Depending on the local temperature, ice may melt quickly. (SM 164)
A fórmula pode existir na matriz ou em outro lugar; isso seria a critério do GM.