Yes you can, as long as they are your own sites or related to you (e.g. a spouse or family members, or a site for a non profit you may be on the board of) AND you are not billing someone else for the work you do on the site or the hosting of the site, you can host several different sites within one hosting plan. To do this, login to the control panel and click on 'Add-on Domains'.  In this section you can configure the domain, the folder/username you want to create for the site and a separate FTP password that you can use if you have other people working on the site.