Yes, there are lots of way you can ensure that a specific combination of input values appears in a generated test case.
For example, suppose you have something like the following in your input definition.
<Var name="protocol">
<Value name="SMTP"/>
<Value name="HTTP"/>
</Var>
<Var name="charSet">
<Value name="Latin-1"/>
<Value name="UTF-8"/>
</Var>
Probably the best technique is to create a generator definition file (<project>-Generators.xml) that demands all pairs of protocol and charSet values. For example:
<Generators>
<TupleGenerator function="*">
<Combine tuples="2">
<Include var="protocol"/>
<Include var="charset"/>
</Combine>
</TupleGenerator>
</Generators>
This will create at least one test case that combines protocol=SMTP and charSet=UTF-8, as well as other combinations that might expose other problems due to interactions between these variables.
Alternatively, if you're not very concerned about interactions among other combinations, you could use a simple constraint that always pairs protocol=SMTP with charSet=UTF-8. The following example constrains the combination in one direction, but you could choose to constrain it in the other direction instead. This limits other combinations, but it might be OK for your situation.
<Var name="protocol">
<Value name="SMTP" property="smtp"/>
<Value name="HTTP"/>
</Var>
<Var name="charSet">
<Value name="Latin-1"/>
<Value name="UTF-8" when="smtp"/>
</Var>