Dynamic MDX query results in same field name but requires different Field Source

I have the following query that returns the results I need but wont work in SSRS

WITH MEMBER [Measures].[Previous Period] as

( 	[Measures].[Fact TOL Activity Count] ,
	ParallelPeriod(
       		IIF(@GroupBy = 'Y',   [Date].[Fiscal].[Fiscal Year],IIF(@GroupBy = 'Q' ,  [Date].[Fiscal].[Fiscal Quarter], [Date].[Fiscal].[Fiscal Month]))
        , 1
        ,[Date].[Fiscal].CurrentMember ))


MEMBER [Measures].[Current Period] as [Measures].[Fact TOL Activity Count]

MEMBER [Measures].[CPeriod] as [Date].[Fiscal].CurrentMember.Name

MEMBER [Measures].[PPeriod] as
	ParallelPeriod(
          IIF( @GroupBy = 'Y',   [Date].[Fiscal].[Fiscal Year], IIF(  @GroupBy = 'Q' ,  [Date].[Fiscal].[Fiscal Quarter], [Date].[Fiscal].[Fiscal Month]))
        , 1
        ,[Date].[Fiscal].CurrentMember ).Name

MEMBER [Measures].[Growth] as

	IIF (ISEMPTY([Measures].[Previous Period]), 0 ,
	[Measures].[Fact TOL Activity Count]/ [Measures].[Previous Period] ), FORMAT = "Percent"


SELECT
{
         [Measures].[Previous Period], [Measures].[Current Period],  [Measures].[Growth], [Measures].[PPeriod],[Measures].[CPeriod]

} ON COLUMNS,


(  TopCount(
    	Except({

  		CASE  [Dim User Type].[User Type Key].CurrentMember.NAME
        		WHEN "Adviser" THEN  [ADVISER].[ID Key].children
        		WHEN "Firm" THEN  [ADVISERGROUP].[ID Key].children
        		WHEN "Investor" THEN [INVESTOR].[ID Key].children
        		ELSE [INVESTMENTMANAGER].[ID Key].children
		END

		} ,

  		CASE  [Dim User Type].[User Type Key].CurrentMember.NAME
       			WHEN "Adviser" THEN  [ADVISER].[ID Key].[Unknown]
        		WHEN "Firm" THEN  [ADVISERGROUP].[ID Key].[Unknown]
        		WHEN "Investor" THEN [INVESTOR].[ID Key].[Unknown]
        		ELSE [INVESTMENTMANAGER].[ID Key].[Unknown]
		END
	)

  , 10
  , (   [Measures].[Growth],  STRTOMEMBER(@CurrentPeriod, CONSTRAINED), [Dim EVENTTYPE].[IASEVENTTYPEKEY].&[ET00000040], STRTOMEMBER(@UserTypeKey, CONSTRAINED))
       )) on rows


FROM  [TOL Metrics Usage]

where  ( [Dim EVENTTYPE].[IASEVENTTYPEKEY].&[ET00000040],  STRTOMEMBER(@UserTypeKey, CONSTRAINED), STRTOMEMBER(@CurrentPeriod, CONSTRAINED) )

The problem is the TOPCOUNT on the rows. INVESTOR, ADVISER and ADVISERGROUP are all actually the same dimension linked to different fields in a fact Table, ie for Adviser it would be Entity.IDKey - > Fact.AdviserKey or for Investor its Entity.IDKey -> Fact.InvestorKey

So using a case statement I can retrieve the correct dimension based on user selection.

However I do not know how to get it to work in SSRS. The field source specifies the exact type and because the field name is the same for all, IDKey, I cannot manually add more field definitions.

Is there a way to dynamically change the Field Source or a better way to do this.

Thanks

Jon