DbCombo editing and Crystal reports in Control arrays

I have a DbCombo box, bound to a datafield in its datasource. If an item not currently in the DbCombo list is to be added to the database
,through the edit portion of a bound DbCombo box,How can it be done? Can you add a new record to the list by adding a bookmark to the existing ones using VisibleItems property?
Crystalreport:
Can control array of Crystal reports be created?How do I submit a SQL filter query to database through Crystal reports at run time?Does Crystal report control send queries to Snapshot type recordsets only?If not how can it be otherwise
Can Crystal report do the following?
For my purposes I require to set up a code which would do something like :
{( Amount) + (x% of Amount)} + y% of {( Amount) + (x% of Amount )} + z% of [y% of {( Amount) + (x% of Amount)}]
I wish that I should be able to choose endless such x,y,z ,... rates at run time to arrive at a final amount