SysML does not provide explicit built-in language constructs to model variants. Nevertheless SysML is useful to create a model for variants. The VAMOS method presented in the book Variant Modeling with SysML from Tim Weilkiens is one option how to model variants with SysML. It uses the profile mechanism of SysML to extend the language with a concept for variant modeling.
The VAMOS stereotypes are part of the SYSMOD profile. You can download the profile and sample models from the books for Cameo Systems Modeler/MagicDraw on the Download page.