Multiple sites onto a single IP

I have an IP associated with my main domain and then I also want a sub-domain to use that IP but when I create a sub-domain it gets assigned to the servers shared IP (I guess). I can’t seem to be able to remove that sub-domains originally assigned IP and change it to the IP I pay for that belongs to my main domain. I know I can pay for and assign a new IP for that sub-domain but the reason I am paying for a unique IP is so that I can assign multiple vhosts to it, some controlled by non-Dreamhost nameservers.

Is it somehow possible to assign multiple (sub)domains to my own paid for unique IP?

No, a unique IP can only be used by one domain. Otherwise it wouldn’t work properly for secure sites, which are the most common use for unique IPs.