Multisite Plugins on non multisite install?

Hi Guys, my site is not a multisite install although it will be handling many thousands of users eventually. I see most of the functionality I want is in Multisite plugins only. Is there a way to get some of these plugins to work for a single install site with multiple users?

The reason I am using a single install is because of the theme and software package I am working with. It was designed to be a full front-end site install but not multi-site. I realise it is not the best choice but I am stuck with it.

Thanks for any suggestions or pointers.