Essentially, you would need to cancel the subscriptions with the provider, and then subscribe again through Roku. (Locate the subscription channel and then subscribe). Just verify that all your desired subscriptions are available to be subscribed through Roku.
You should realize that if you choose to consolidate and pay Roku for your subscriptions, you will only be able to view them through a Roku device (and some through Roku mobile app or through the Roku Channel on a computer).
Most people prefer to subscribe directly to the provider so they can use the subscription on any device and not be limited to watching on a Roku device or through the Roku Channel. However, if the majority of your watching is only on your Roku devices, then consolidating may be more convenient to have all your subscriptions in one place.
Roku Community Streaming Expert Just another Roku user... I am not a Roku employee. Insignia RokuTV, Ultra 4660, Premiere+ 3921, Express 4k+ 3941, Streambar 9102