Technically possible? Yes – the user can give someone a (short-lived) Gaia association token that will allow that someone to write data to their app bucket on their behalf. That someone could be the Radiks server itself.
Supported in Radiks today? I don’t think so – not directly. You’d need to have the user generate an association token, and then have this other agent use it to generate a custom Gaia hub config with connectToGaiaHub(), and then bind it to its own UserSession with getOrSetLocalGaiaHubConnection(). From there, that agent’s UserSession can write to the original user’s Gaia hub under the constraints given in the association token.