As per product functionality, you cannot add expression measures into a computed measure since computed measure are generated run time & being created by ARC once data has been fetched from database.
Expression measure can be used where a conditional statement (for example, IF-THEN-ELSE) is necessary. You can use a base measure or a computed measure within an expression measure. An expression measure cannot be used within an expression measure.
Expression measure is used in instances where there is a necessity to satisfy conditions using certain conditional statements like IF-THEN-ELSE. A base measure as well as a computed measure can be used/referred within an expression measure; however ...