The CDS Certificate Policy (available here) specifies in section 6.1.1
Subscriber key pairs must be generated in a manner that ensures that
the private key is not known by anybody other than the Subscriber or a
Subscriber’s authorized representative. Subscriber key pairs must be
generated in a medium that prevents exportation or duplication and
that meets or exceed FIPS 140-1 Level 2 certification standards.
SoftHSM is not FIPS 140-1 Level 2 compliant. As far as I know, level 2 and upper requires a hardware cryptographic module.
Note that the AATL program (successor of CDS) has similar requirements but it provides a larger set of Certification Authority which could provide you either a physical signature certificate or a signature SaaS solution.