There is a way to do it, but it's not nice - you have to modify the JSP file that Aware IM generates which means that you have to re-apply the changes every time you publish a new version.
Just a thought, but could you use an i-frame in the custom page which would provide you with scrolling automatically without having to modify the jsp file. It is possible to add an iframe with 100% width and 100% height.
There must be a file in the AwareIM/Tomcat/webapps/AwareIM directory that starts with BAS_Welcome, which is followed by the name of the business space.
Open this file and search for scrolling="no". Then replace it with scrolling="yes"