It's very simple to have package that depends of another in its own channel like your (pear.firephp.org)
Here is a bit of an example to build / prepare a package (with PEAR_PackageFileManager, if you used it)
and add an external dependency to a resource in another channel
Suppose we have to build the PEAR::Log package, and add the firephp driver
<?php
require_once 'PEAR/PackageFileManager2.php';
$p2 = new PEAR_PackageFileManager2();
...
$p2->addPackageDepWithChannel('optional', 'FirePHPCore', 'pear.firephp.org', '0.1.2');
Will produce something like in package XML 2.0
<dependencies>
<required>
<!-- ... -->
</required>
<optional>
<package>
<name>FirePHPCore</name>
<channel>pear.firephp.org</channel>
<min>0.1.2</min>
</package>
</optional>
</dependencies>
