You're asking some strange questions. What are you wanting to do? You say you're a moderator of a forum, but who would give moderation privilages to someone that, not trying to offend here, doesn't know the basics. Tell us exactly what you are trying to do and we can go from there. Per your last question, no you could not do that, but why would you want to even if you could? Just looking at your screen name gives me a funny feeling about this entire conversation.
The way your dynamic IP addresses works goes like this - Your ISP assigns you an IP address that changes from time to time, this makes it dynamic. The rate at which it changes it variable and depends on a few things. The two main factors are the lease time assigned by the ISP in their DHCP server and whether or not your DSL/Cable modem is powered on and online during that time frame that your IP address assigned by the ISP is set to expire.
If I had to venture a guess, you have banned someone via IP address or subnet from somewhere (or you yourself have been banned) be it a game server or a forum and now you're wanting to know if they/you can get around that. I may be wrong about this so forgive me if I am.