Hey,
I'd say via the SDKs would be the easiest/cleanest (& potentially best).
But if you can't, I have managed to do it at some point from a file import to ACS with tokens, Name of the App & Platform (ios, android) + any other data required (if you extended the appsubrcp resource).
In terms of loading the data, its kind of a 2 step process:
1. Load data to the AppSubRCP, need to do a enrichment of data linking the incoming data to existing Apps (using the value in your file & @2175539 in Mobile Applications, that way you can update the mobileApp link). I also loaded the first 10 Characters of the token to the marketingcloudID field.
2.At the same time, load the same data to the PushNotificationStatus resource (just token & marketing cloudID). Similar to point 1, I loaded the first 10 Characters of the token to the marketingcloudID field.
Point 2 seems to be needed, otherwise you cant' actually view or see the appsubrcp data.
Cheers,
Don