Dreamhost CAN'T hold the registration for .pro (or .co.uk or 101 other domains)
Dreamhost CAN host the web-sites for them...
i have 9 domains, 8 of which are registered via 123reg, 7 of these are .co.uk's and i changed the nameservers to ns1.dreamhost.etc
then you set up domain through "Manage Domains -> Add New Domain -> www.domain.pro" and dreamhost will host the domain BUT the registrar holds the actual domain registration... not dreamhost
does that make any more sense?