Is it permissible to return stolen money by putting it in the Zakat fund?

It is forbidden to trespass on another person's network and use it without their permission, due to the sanctity of a Muslim's property. However, it is not considered theft in the full شرعي (legal/Sharia (definition)) sense. The one who used it must repent to Allah and seek absolution from the rightful owner by compensating them or asking for their forgiveness. It is not a condition to inform them of what you did when returning the right. Furthermore, one's obligation is not discharged by placing the money in the Zakat (definition) box except when unable to reach the rightful owner.