Do you mean that you want to assign a value from within a shell variable to an N1 variable? If so, then you can use the <assignOutput> element in execNative. Here's an example:
...
<varList>
<var name="var1" default="not assigned"/>
</varList>
<simpleSteps>
<execNative>
<assignOutput varName="var1"/>
<exec cmd="echo">
<arg value="${my_shell_var}"/>
</exec>
</execNative>
<raise message=":[var1]"/>
</simpleSteps>
...