We do currently use Hudson for our continuous integration server. Can buildmaster connect or co-exist with that or would we need to replace it with buildmaster ?

Yes, Hudson (or another CI server) can definitely co-exist. Some of our customers simply have BuildMaster pickup artifacts or build output from another tool like Hudson and use BuildMaster to deployment and implement the approval processes.

However, BuildMaster does have CI capabilites (see Build Schedules), and can completely replace your Hudson/CI tool in the future