Use random string variable in a insert
insert into dbo.tblexecution (
     fname
    ,lname
    ,brtday
)
select top 100 
     [FirstName]
    ,[LastName]
    ,convert(date, dateadd(day, (abs(CHECKSUM(newid())) % 3650) * - 1, getdate()))
from [AdventureWorks2008R2].[Person].[Person]
order by newid()