Bottom line is you would probably be best served by decommissioning the less capable small cost reduced DC power supply and using an AC power supply in its place. A CW-80 would get you there for a single loop of track and an accessory or two. A PS180 with or without the GW180 would give room for growth.
Also, don't overlook a serviced Postwar Transformer like an RW, LW, KW or others.
Doorbell transformers are such low current, they are unlikely to be capable of performing the task of operating all the features of the UCS.
An auxiliary power supply can be used by the UCS by powering the controller through the wire that would have instead gone to the 3rd terminal (Left to right) provided that you have common on the outside rail.