The description of the SUBDIRS variable in the qmake Manual states the following:

It is essential that the project file in each subdirectory has the same name as the subdirectory itself, so that qmake can find it. For example, if the subdirectory is called myapp then the project file in that directory should be called myapp.pro.

http://doc.qt.nokia.com/4.7-snapshot/qmake-variable-reference.html#subdirs

However, it seems to be possible to specify a relative path to a .pro file in any directory.

For example, the following code specifies that plugin_coreplugin/plugin_coreplugin.pro and mylogin.pro belong to the project. For example,

TEMPLATE = subdirs

SUBDIRS = plugin_coreplugin \
../another/plugin/myplugin.pro