I'm not absolutely sure, but since nobody else is answering here's my take.
The only entity which is able to sign certificates is a CA. There are different levels of CAs, so you could in theory set up your own CA subordinate to CaCert (and hence have it have its own CA cert signed by CaCert). This would make normal certificates you would be signing participate in the chain of trust of length 3 (rather than two).
From what I gather from this page, it's somehow possible to become a "CACert member" and get what they call "subroot" certificate — this one would allow you to sign your own certificates and make them trusted by anyone trusting the root CACert certificate (provided you also make available the certificate of your subordinate CA in one way or another — for instance, a server using PEM-formatted certificates might use a certificate file which is merely a concatenation of both the server's certificate and the CA certificate in PEM format).
My personal experience with this kind of setup was this: some time ago the xmpp.net federation provided paid-free certificates for any XMPP server whose admin wished to get one. That federation was itself a CA subordinate to StartCom. So after getting my server certificate I needed to tell my server to present both its own cert and the xmpp.net cert in one bundle to make the trust chain complete for its clients as they would usually trust StartCom but not xmpp.net.