Write a value identified by a key... Existing values and expirations will be overwritten. If neither expiration nor expiration_ttl is specified, the key-value pair will never expire. If both are set, expiration_ttl is used and expiration is ignored.
PUT accounts/:account_identifier/storage/kv/namespaces/:namespace_identifier/values/:key_name?expiration=:expiration&expiration_ttl=:expiration_tt
Looks like this isn't supported currently in the lib: https://api.cloudflare.com/#workers-kv-namespace-write-key-value-pair