Hi,
Unfortunately migrate2rocky doesn’t have an option to be able to use local repos. You would have to modify the script to be able to do so. A quick summary of what would need modifying:
The Rocky Linux GPG Key would have to be pre-downloaded and migrate2rocky modified to use that key instead of attempting to download the key itself (around lines 141 and 1279).
The repo URLs for baseos and appstream need to be modified to point to a local copy of those repositories (line 149)
The rocky*.repo files need to be modified after they are installed to point to the local copies of the repos by adding code to do so around line 1098 (before the distrosync).
In addition to this you need to have a working local copy of the CentOS Stream 8 repos from vault and have the appropriate CentOS-Stream-*.repo files modified to point to those local copies. You should not have to modify migrate2rocky for this step but in case you get issues it may help to modify all the centos-stream URLs from line 623.
I believe that this is all the places that need modifications but keep in mind that there may be more and it would be very easy to make a mistake which screw up the migration so make sure that you have a full backup as well as a snapshot that you can easily roll back if you decide to attempt this.
Otherwise it may be easier to just temporarily allow a proper internet connection through to the target machine and then change it back once your migration is done.
Good Luck,
Peter