If I'm not wrong, It's not possible. In short, if you decide to change some part of web2py that's fine but you may lose some cool stuff.
Since, SQLFORM expects a TABLE object, you can't use your SQLAlchemy objects here. However, You may found formencode <http://formencode.org/> useful (for form validation and processing) There are a few others but I can't recall them right now.
