Group by function when query includes an embedded Select statement

This is more of a SQL question than specific to BI Publisher.  I am trying to summarize cost by month to use in a pivot table.  I'm using the following query:

select  "PROJECT"."NAME" as "PROJNAME",

           "ACTIVITYEXPENSE"."EXPENSEITEM" as "EXPENSE",

           (select to_char("UDFVALUE"."UDFDATE", 'YYYY-MM') as "MONTH"

               from  "PXRPTUSER"."UDFVALUE" "UDFVALUE"

               where "ACTIVITYEXPENSE"."OBJECTID" = "UDFVALUE"."FOREIGNOBJECTID" ) as "EXPYYYYMM",

           sum ("ACTIVITYEXPENSE"."PLANNEDCOST") as "PLANNED"

from  "PXRPTUSER"."ACTIVITYEXPENSE" "ACTIVITYEXPENSE",

          "PXRPTUSER"."PROJECT" "PROJECT"

where   "PROJECT"."OBJECTID"="ACTIVITYEXPENSE"."PROJECTOBJECTID"

group by "PROJECT"."NAME", "ACTIVITYEXPENSE"."EXPENSEITEM", "?????"

The ????? at the end is where my question begins.  Can I group by the "EXPYYYYMM" field created by the select to_char function?