Thelma,
so if I understand you correctly your plan is to use an OpenACS/dotLRN web server and a Tomcat web server in parallel. I don't know if there is experience with such a setup but I suspect it would be a quite tedious endeavour. Writing your app within the OpenACS framework is most likely a better option. Yes, you could reuse the OpenACS datamodel from your Java server, but remember that a lot of the power of OpenACS is embedded in the Tcl layer.
If you necessarily want to use Java (why?) you are probably better off changing altogether to an open source Java groupware/e-learning framework. Unfortunately, I don't know of any such frameworks that would offer a compelling alternative to either OpenACS or dotLRN.
