While it may be technically possible to use ADMe for this, any sort of large scale conversion between media is likely going to take far too long to be practical. If you're only migrating yearlies or monthlies, that might be possible but I don't think it would be practical to migrate daily backups with 7 year retention.
We are only migrating yearly backup while monthly and daily we will keep in old networker until it expired. Each tape (5 alltogether) only have around 300GB of data.
Unfortunately the requirement here is for the opposite of what ADMe provides that being migrating Avamar BU's to tape so ADMe is not suitable here.
However, I believe you could manually emulate a similar approach to how ADMe performs migrations and given the data size and number of BU's involved here is small it wouldn't take long to accomplish.
1. Provide a staging disk area on a dedicated drive letter large enough to handle one or all BU's as one migration. If different retention's are needed for each migration once in Avamar, then migrate just one BU from Networker at a time. The staging disk can be on a VM or any existing server to which you will then perform a directed recovery from tape to it.
2. A manually created staging path should be established on the staging drive letter similar to the example below used to differentiate each BU being migrated and its original BU date in Networker in 3rd folder.
F:\NWK\<original-NWK-client-name>\<YYYY-MM-DD>\<your directed recovery from tape>
Note: you may need to recover from NWK on a per drive letter basis as I'm not sure how NWK distinguishes between drive letters when more than one is being recovered at a time to a common path
3. Once data is staged, use Avamar GUI to perform a simple interactive FS BU from the staging server & path used. You can assign the required retention value as part of the new Avamar BU or you can assign it after the BU to Avamar is completed.