It's not possible for now. There is not attributes for Target for this purpose.
I will explain you my workaround:
I have a folder bin/targets (avoid to use bin/phing if you are using composer to handle phing dependencies) with several xml files:
bin/targets/
├── db
│ ├── default.xml
│ └── one_database.xml
├── geonames.xml
├── jmeter.xml
├── qcode.xml
├── skel.xml
├── symfony.xml
└── test.xml
Each one has several targets, and all of them are prefixed with the "namespace". In example, target with name db.one_database.deploy is in this file ./bin/targets/db/one_database.xml,
At the end of my build.xml file I have these instructions
<import file="bin/targets/skel.xml" optional="false" />
<import file="vendor/corretgecom/phing-base64/bin/phing/qgpl/corretgecom.qgpl.base64.xml" optional="false" />
<import file="bin/targets/symfony.xml" optional="false" />
<import file="bin/targets/test.xml" optional="false" />
<import file="bin/targets/qcode.xml" optional="false" />
<import file="bin/targets/jmeter.xml" optional="false" />
<import file="bin/targets/db/default.xml" optional="false" />
<import file="bin/targets/db/one_database.xml" optional="false" />
When you execute bin/phing -l targets are not in separated groups but namespaced and sorted by name :)