Hi,
Unfortunately, modifying the merge process is quite challenging because 3ds Max manages references automatically. For example, if an object is replaced by another with the same name, it will not be reassigned automatically. To avoid any issues, the ForestPack plug-in adds prefixes to ensure that all imported objects have unique names. However, since the Forest Pack 8.2.2, you can use the Library Browser > Preferences > 'Use unique names on imported objects (add prefix)' option. By default, library objects are imported in the 'forest_templates' layer, with a unique name as '_forest_library_XXXX'. This option lets you disable the prefix and import the objects with their original name. Use with care - disabling it would overwrite existing objects in the layer with the same name, but in some situations - like yours - you can find it handy.
I hope that helps.
Best regards,