Can I add another addendum to the lease agreement 3 months into the contract and require signatures from all roommates?

Current lease agreement has an addendum that states zero tolerance for illegal activity. Landlords include myself and daughter, with daughter living in the townhouse with 3 other roommates. Two roommates have bongs that they keep in the townhouse that landlord was unaware of. Landlord has requested removal of bongs from property as they are paraphernlia to illegal activity. My concern is that I should have this stated specifically in an addendum. "No illegal activity or paraphernelia is allowed in townhouse or garage." Can an addendum be added to the lease agreement and require signature of all roommates? Thank you.