Hi Hans,
As others have already said, the only type of file that you can load into the keyboard is a 'pack install file' with extension '.ppi' or '.cpi' - you can't load '.ppf' or '.cpf' directly. Purchased packs are usually supplied as project pack files (.ppf/.cpf) and may also include install files too (.ppi/.cpi), but the most important type for most people are the project pack files which can be imported into YEM.
Each time that you load an install file into the keyboard, the expansion memory is formatted and all existing custom voices/styles etc are deleted from the keyboard.
The idea is that you import the project files of all your packs into YEM, and then use it to make a install file containing voices and other content of your choice from different packs, up to the maximum amount of expansion memory in your keyboard (approx. 768 MB on a PSR-S975). If you want to add extra content later, then you use YEM and select all the currently loaded content plus the new to include in the install file assuming that all the voice wave samples can fit in the available memory). Note that install files can't be imported into YEM and you can't recover the expansion content currently loaded in the keyboard to any type of file.
I assume that the 'Russian' pack you refer to is the 'Symphonic Music' pack and if so, this only contains styles. It is available for free from Yamaha so you don't have to worry about loosing that. Here is link to the page on the Yamaha UK website from where it can be downloaded...
https://uk.yamaha.com/en/products/musical_instruments/keyboards/arranger_workstations/psr-s975/vse.html#product-tabsThe styles from the pack will probably be in the 'Expansion' folder in the User drive at the moment, and if so, you should be able to 'save' each of the 20 styles from there to USB if you prefer. They will play directly from USB.
However the 'Church & Christmas' pack does contain voices and is not free, so unless you were supplied with a copy of the pack to import into YEM (perhaps on a USB stick), you will need to contact the person who sold you the keyboard and request a copy of the .ppf or .cpf version of the pack if you want to continue to use it with other packs.
Regards
Ian