mojoPortal pages enforce role security with its own code but cannot affect non-mojoportal custom page code.
However, ASP.NET has ways you can protect that page based on roles. You can put another Web.config file (a minimal one, not a copy of the mojoportal Web.config), and from there you can set allowed roles in the <authorization section.
There are lots of tutorials on the web if you search for "protect folder forms authentication" or "forms authentication location", these are example articles that should help.
http://www.codedigest.com/CodeDigest/14-Controlling-Access-to-Folders-in-FormsAuthentication-using-location-Element.aspx
and
http://www.theserverside.net/tt/articles/showarticle.tss?id=FormAuthentication
note that you could do it from the mojoPortal Web.config using a <location element, but the advantage of just using a separate Web.config file in your folder is that you don't have to maintain this customization in the mojoPortal Web.config during upgrades.
Hope it helps,
Joe