Hi Manuel.
Currently there is no supported (or indeed technical) way to physically 'deploy' an XPages application to a Portal Server (that I am aware of). You should investigate using 'iWidgets'. iWidgets will allow you to make an xpage into an artifact that is consumable within portal, and can participate within the portal framework (e.g. publish and receive events & data).
In Domino Designer iWidgets are called 'components', under the composite applications branch in the navigator. In your existing XPage application simply create a new Component, that component can then be consumed by the Portal Server which will result in the XPage being rendered within Portal.
Hope this helps.
Dan