Are there any existing ASN.1 structures for transferring symmetric keys?
For the algorithm you should use the AlgorithmIdentifier type of RFC5280. Block size and key size can be derived from the algorithm id and the key itself, so there is no need to store them. 

So, if you want something really lightweight, just store that into a sequence:

SymmetricKey ::= SEQUENCE {
  algorithm AlgorithIdentifier,
  key       BIT STRING
}