stored procedure - please advise

i'm trying to set a variable to equal the results set of a stored procedure. please advise me on what is wrong with my "set" statement.

declare @categoryName varchar(50)
set @categoryName = 'MAIL - OVERALL'


DECLARE OUTSIDE_CURS CURSOR FOR

select uptimeEvent.system_name
FROM UptimeSystem RIGHT OUTER JOIN
UptimeEvent ON UptimeSystem.system_name = UptimeEvent.system_name
where category = @categoryName and type = 1
group by uptimeEvent.system_name

FOR READ ONLY

OPEN OUTSIDE_CURS

DECLARE @SYSTEM VARCHAR(55)

FETCH NEXT FROM OUTSIDE_CURS INTO @SYSTEM

WHILE (@@fetch_status <> -1)

BEGIN
IF (@@FETCH_STATUS <> -2)
BEGIN

DECLARE @SUBSYSTEM VARCHAR(150)
SET @SUBSYSTEM = EXEC dbo.sp_PHS_UPTIME_PERC @SYSTEM
PRINT @SUBSYSTEM

END
FETCH NEXT FROM OUTSIDE_CURS INTO @SYSTEM

END

CLOSE OUTSIDE_CURS

DEALLOCATE OUTSIDE_CURS

Is it on sql server? You can't assign value to variable with exec.