If FreeBSD ports fail to build on DragonFly they don't get included in the dports repo. There may already be logs here.
Copy the broken port from your fresh FreeBSD ports tree into your dports tree. Now you can try building it on DragonFly and fix what's broken.
Submit a fix to FreeBSD ports or dports, depending on what the fix is. Ask on the lists or in the IRC channel if you don't know the appropriate place. dports issues are reported to the dports issue tracker and ports issues go to FreeBSD's send-pr.