Some github docs about transfer.
And as Matt pointed out, being a member of InsightSoftwareConsortium should be enough to transfer to it from a personal account.
But the existing fork in ISC should be deleted first @dzenanz@matt.mccormick InsightSoftwareConsortium/ITKIsotropicWavelets already exists
Before you transfer a repository, keep these things in mind:
The target account must not have a repository with the same name, or a fork in the same network.
Maybe you can fork ISC/ITKIsotropicWavelets into your account temporarily and delete the renamed fork in ISC?