So SSH into your Synology box:
ssh root@127.0.0.1
Obviously you may have a different user IP address and if you have specified a different port you will need to add it in:
ssh -p 45 user@127.0.0.1
Once logged in you can simply copy the file to a folder that you can access:
cp /etc.defaults/ddns_providers.conf /yourfolder
Then you can open the file in your favourite editor
copy the provider you want multiple entries for and just rename them _1 _2 etc…
Save the file and then copy it back to the original location
cp /yourfolder/ddns_providers.conf /etc.defaults
And you should now have the ability to choose more than 1 instance of the same provider.
BEWARE: sometimes DSM updates reset this file to its orginal state so you might need to re-copy your file after an update.