Technically speaking you can sign whatever you want with your eID card. Anyone with your certificate will be able to validate the signature against your public key and validate the certificate path (assuming that the verifier considers the Citizen Root CA consider as a valid Trust Anchor).
However signing another certificate with your eID is not a good practice for several reasons.
First of all, it does not respect the X.509 standard. I mean that if you sign a certificate with your eID key, the validation of this certificate will be rejected by any conform X.509 validator. I see at least two reasons:
- the Citizen CA Certificate contains an extension (named Basic Constraints) which explicitly specified that it can only issue End User certificate, i.e. certificates which are not allowed to sign other certificate
- the eID citizen Signature Certificate contains an extension (named Key Usage) which explictly indicates that the certificate signature purpose is forbidden.
Moreover these restrictions on the certificates only reflects the certificate issuance rules which is called the certification policy. The certification policy is a document which indicates how the Certification Authority issues the certificates (for instance how the certificate holder identity is verified, which information are included into the certificate, the acceptable and prohibited usages of the certificate...).
The eID certification policy is available here: http://repository.eid.belgium.be/downloads/citizen/en/CPS_CitizenCA.pdf
The section 4.5.1 Citizen duties says:
Only using certificates for legal and authorised purposes in accordance with the CPS.
It means that even if a signature is technically valid, the value of this signature is subordinate to an authorized usage of your certificate according to the policy. In other term it is technically feasible to sign a certificate with your eID card but, since this usage is not authorized by the policy, this certificate has no value in the Citizen CA context.
But note that instaed of creating a new public key what don't you directly use your eID citizen Signature Certificate?