You need to define a Dynamically Calculated member, usually in UD8, that retrieves the opening date value. Being non-cube data, you would normally surface the value as V#Annotation.
Now, onto how you are retrieving the opening date.... While you can build the dynamic calc member to do just about anything, you should be considering performance, as that dynamic calc member will need to be invoked for every Cube View cell that is to display the value. So, if you have 1000 stores in the rows, you will have to retrieve 1000 opening dates - one-at-a-time. Every time the Cue View is refreshed, that is all done again.
The reason I mention that is that unless you can cache that opening date table into in-memory cache, that means for each opening date you need to surface, you need have your dynamic calc formula open a connection to the DB and issue a SQL statement to fetch the specific opening date value you are interested in from your custom table. That is VERY expensive to do.
A far better option would be to set the opening date into a Store member Text[n] property. Rather than reading directly from your custom SQL table, you would have your dynamic calc retrieve the value for from the member's Text[n] property. OS caches all the metadata into in-memory cache for use with the api functions. So, as long as you use the OS api functions rather than the BRApi functions, this method can get the opening date info cheaply.
