You don't tell us what keyboard you have, that would really help!
You'll have at least one tremolo strings voice. Look through what you have and if you can, layer two of them. This is almost essential for realistic tremolo (or tremolando) strings, as in a real orchestra, unless the music specifically requests a 'measured tremolo', the players will all play the tremolo at slightly different rates. The tremolo strings voice captures this but it's still artificial. Layering two different tremolo strings means that the tremolos will overlap and weave, resulting in a more realistic sound.
Sfz and tremolando are two very different things. It would be odd for an sfz voice to have tremolo. An sfz voice will usually start loud, drop down suddenly and then come gradually back up. But I've not tried any sfz strings recently.
With great respect to panos, adding a tremolo to a strings voice, while it's an interesting sound, is not the same as tremolo or tremolando.