Is it permissible for an employee of a charity to use a computer and internet network bought by the charity for its benefit, for his personal use, knowing that the subscription cost is fixed and does not change with his usage?

It is permissible to use a device owned by an entity if the owning entity permits it, and it is not permissible without their permission. It is also not permissible to stipulate private work on the entity's devices unless the work is known and clear, in which case it would not be voluntary work.