Current syntax: add the following to your own extension's constants file and load it after Powermail's own. The same syntax applies to partialRootPath and layoutRootPath.
You only need to copy the individual file you want to customize - respecting the original folder structure - and not the entire folder. TYPO3 will search for the file in all of the paths indicated in the ...rootPaths array, starting with the highest-numbered entry.
By using the singular form ...rootPath and not ...rootPaths, your definition will be appended to the generated ...rootPaths array. Therefore, adding the following to your constants…
plugin.tx_myextension {
view {
templateRootPath = EXT:myextension/Resources/Private/Templates
}
}
…will generate…
plugin.tx_myextension {
view {
templateRootPaths {
0 = EXT:powermail/Resources/Private/Partials/
1 = EXT:myextension/Resources/Private/Templates
}
}
}