Short Theoretical Answer:
In theory SysML and UML model elements can be synergistically combined in the same model. Indeed, this was one of the language design goals of the SysML Partners
' open source specification project:
Longer Practical Answer:
• Support SysML + UML mixed language usage:
Ensure that SysML constructs can be synergistically combined with UML constructs in a model shared by Systems Engineers and Software Engineers, where the former use SysML and the latter use UML. The synergistic combination of SysML and UML should maximize requirements traceability and minimize semantic overlap between the two languages.
— SysML Language Design Goals, SysML Partners
In practice, unfortunately, users frequently find it confusing and problematic to combine SysML and UML model elements in the same model, just as they find it befuddling and vexing to interchange SysML and UML models across supposedly XMI (XML Model Interchange) compliant modeling tools. It appears that the Object Management Group (OMG) is following a "family of languages" visual modeling design philosophy, as opposed to the "unification of languages" design philosophy that dominated UML 1.x adoption and revisions during the late 1990's and early 2000's. This misguided and poorly executed design philosophy has resulted in a "Tower of Techno-Babble" anti-pattern for users who want to synergistically combine the OMG's most popular visual modeling languages (UML, SysML, BPMN), and it has resulted in a "Meta-Babble" anti-pattern for vendors relying on the gratuitously complex Meta Object Facility (MOF) and XMI for reliable model interchange.
So, given the "Tower of Techno-Babble" anti-pattern described above, how can a team comprised of both Systems Engineers and Software Engineers specify a model that harmoniously combines SysML and UML model elements? The following outlines a technical approach based on the transitivity of requirements traceability dependencies (notably «satisfy» and «refine»), that has proven effective on numerous large, mixed modeling language projects: