Are there any existing ASN.1 structures for transferring symmetric keys? Is there any reasonable ASN.1 standard to encode symmetric keys?

I found PSKC, but it seems extremely heavy, and I can't quite find any library (e.g. bouncycastle) that would simplify using it.

If all I need is to encode a symmetric key, would it be sufficient to just include:


algorithm ID
block size
key size
key bits
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
}