I found a solution to my problem here.
The resulting xml looks like this:
<?xml version="1.0"?>
<ruleset name="My PMD ruleset"
xmlns="http://pmd.sourceforge.net/ruleset/2.0.0"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://pmd.sourceforge.net/ruleset/2.0.0 http://pmd.sourceforge.net/ruleset_2_0_0.xsd">
<description>
My PMD
</description>
<rule ref="rulesets/java/naming.xml/ShortVariable">
<properties>
<property name="xpath">
<value>
//VariableDeclaratorId[(string-length(@Image) < 3) and (not (@Image='id'))]
[not(ancestor::ForInit)]
[not((ancestor::FormalParameter) and (ancestor::TryStatement))]
</value>
</property>
</properties>
</rule>
</ruleset>
To be able to ignore more variable names, repeat the following part:
and (not (@Image='myVariableToIgnore'))