Support for SAMLAssertionID key identifiers

I like the signature to reference the certificate within a SAML assertion (SAML Holder-of-Key). Therefore I need to be able to add a key identifier with value type "http://docs.oasis-open.org/wss/oasis-wss-saml-token-profile-1.0#SAMLAssertionID". How can I configure this in SoapUI?

<wsse:KeyIdentifier ValueType="http://docs.oasis-open.org/wss/oasis-wss-saml-token-profile-1.0#SAMLAssertionID">bc22375fb6c25e557bab007f83dfb9c6</wsse:KeyIdentifier>
</wsse:SecurityTokenReference>

I cannot add a signature statically because I need to sign the body and a timestamp.