You can have multiple single app subscriptions on same account.
Please check the pricing for single app: Creative Cloud pricing and membership plans | Adobe Creative Cloud
For Photoshop we have to plans.
1. Photoshop Single App Membership
2. Creative Cloud Photography plan. | with Lightoom 5
For plans check : Creative Cloud pricing and membership plans | Adobe Creative Cloud
Note: if you need more than 3 single, its recommended to purchase Complete Creative Cloud membership.
You may not be able to add Photoshop CC to your current subscription.
Either you can have two different Single App subscription under your account or One Creative Cloud Full App subscription.
Oh I see. Okay, I guess I'll just have to get a seperate subscription for Photoshop then.