Hi lyndaw ,
Lightroom 6 cannot be added to Monthly payment using Cloud subscription as Lightroom CC is already available in the mostly subscribed Photography plan.
However, You could subscribe to Creative Cloud to access other Products.See Lightroom and Photoshop | Adobe Creative Cloud Photography plan
Refer Creative Cloud pricing and membership plans | Adobe Creative Cloud for Creative cloud Plans.
How does Lightroom CC differ from Lightroom 6?