Parameterising the parallel="methods" property in the suite tag of testng.xml

I have a situation where i need to implement a switching mechanism on testng suite, to decide whether to run tests parallely or not.

Currently my testng.xml looks as following:

<suite name="XYZ" parallel="methods" thread-count="3">
    <test verbose="2" name="ABC">
        <classes>
           <class name="xx.yy">
               <methods>
                   <include name="test3" />
                   <include name="test2" />
                   <include name="test1" />
               </methods>
            </class>
           </classes>
    </test>
</suite>

My objective is to be able to specify something like :

 <suite name="XYZ" parallel="$PARALLEL_FLAG" thread-count="3">

where the $PARALLEL_FLAG can take values methods or false dynamically.