I have no direct experience with .eu domains, but DreamHost host .au domains fine, so I imagine .eu domains would be the same.
If you choose not to register a domain during the sign-up process, you are given 1 domain ‘credit’ that can be used at a later time.
As for transferring the .eu domain, I am not totally sure this is possible. DreamHost only allows you to register .com, .net, .org and .info domains through them, but they may (or may not) allow you to transfer a .eu domain registered elsewhere. Your best option would be to ask them and find out for sure.
Of-course, you could always leave the domain registered where it is and host the actual site here at DreamHost. This is achieved by simply configuring the domains DNS to point at the DreamHost DNS servers.
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