how do i enable a clients acount to be able to add parked domains?

what registrar?

^ the registrar is godady.
i've got 3 doamin domain.com domian.co.uk and domian.net

i've created an account for domain.com on my resseller package with Hometownhosting.

all three domians point to hometownhosting name servers.

i just want domain .co.uk and net to point to the same acount "domain.com"

you need to either register that domain with your hosting too (from cpanel) or you are limited to just one domain, use godaddys forwarding service.