From The Creator: You can't extend samples using EGs. You can extend how long they play once you release the key, or cut those to an 'early' point, but you cannot 'create' new information in samples, assuming they are unlooped.
Several samples in Dimension Pro (sax samples, for instance), are unlooped. they last as long as the real sax player could resist breathing and keeping the pitch on a note. This gives you long samples, which make a lot for increased realism. As the wave files are external, it is still possible to create a looped version of any instrument, using a wave editor and... patience.
The maximum amount of time you'll ever get out of a core sample is the length of the sample itself. This is an audio (.wav) 'issue', not an .sfz problem or MIDI workaround.
That said, if you use Sound Forge, and would like some guidance in setting up loop points (within the samples themselves), I can help you to slog through the process. Dimension Pro will 'honor' those loop points, unless overridden by another parameter.