I have solved the problem, in an icky way, but it works great!
<project name="Test" default="main">
<property file="agent.properties" />
<property file="temp_updates.txt" />
<taskdef name="propertycopy" classname="net.sf.antcontrib.property.PropertyCopy" />
<taskdef name="foreach" classname="net.sf.antcontrib.logic.ForEach" />
<target name="main">
<property name="Agent Updates" value="6.1,6.2" />
<antcall target="create_temp_files" />
<antcall target="agent_updates_target" />
<propertycopy name="custom.agent.release.group" from="updates" silent="true" override="true" />
</target>
<target name="agent_updates_target">
<foreach list="${Agent Updates}" target="agent_version_to_path" param="var" delimiter="," />
</target>
<target name="agent_version_to_path">
<propertycopy name="var" from="agent.installer.${var}" silent="true" override="true"/>
<echo message="${var};" file="temp_updates.txt" append="true" />
</target>
<target name="create_temp_files">
<echo message="updates=" file="temp_updates.txt" />
</target>
</project>
on another file, "agent.properties" I had that;
agent.installer.6.3=something3
agent.installer.6.2=something2
agent.installer.6.1=something1
agent.installer.6.0=...
agent.installer.5.6=...
agent.installer.5.0.12=...
agent.installer.5.0.11=...
agent.installer.5.0.9.5=...
agent.installer.3.8=...
agent.installer.3.7=...
As a result, a new file "temp_updates.txt" was created, having
updates=something1;something2;
Which I then loaded into the actual program.
May not be pretty, but it works quite well.
Thank you Skoll and Mark O'Connor for all your help, I used those ideas to come up with this one. I would rate you, but I can't :( Sorry!