Hi,
I’m not quite sure what you want to achieve.
Do you want to migrate from your self-hosted BIND9 to deSEC (and shut down BIND9 when done)?
Or do you want to use both deSEC and your own BIND9 in parallel, possibly synchronising zone data between them automatically?
The former is relatively easy if you can get the zonefile (which I suppose Webmin writes for BIND). I do not think the deSEC API can directly import it but there are several third-party API clients. At least this one can import zonefiles to deSEC:
If you want to achieve the latter, I don’t think this is possible with deSEC. The main issue is that your own nameserver and deSEC’s can not share the private keys with which to sign DNS records. And this means that resolvers will observe inconsistent data between then nameservers.
You would need to find a DNS provider that supports acting as a secondary to your nameserver or allow setting up your nameserver as a secondary to theirs.