Hello Hidden,
I'll assume you are using cPanel on your dedicated box.
You can run a multitude of domains on the shared IP with no problem at all. But if you do have a domain that needs a dedicated IP then in WHM go to Account Functions | Change Site's IP Address. Select the account/domain and click the Change button. In the next screen click the drop box and select the IP you want to use then click Change again.
Hope that helps!