Question: I would like to set a String Expression for an entity to be a value for an extended entity in SalesLogix. Is it possible for me to do this?
Answer: This way may be a bit of work, but you can try it as a workaround:
On the Main Entity, add a New Code Property.
On the Code Property, make it return a String which is the value of your ExtendedEntity.Property (check for null, etc).
On the String Expression, use your new Property, you don’t need to try to link here to the Extended Entity.