Creating a Rollup Summary Field

Creating a Rollup Summary Field

Is it possible to have a field from one module to updated another?

For example
I would like the Account Number from the Accounts Module to equal Account number in the Possibility Field with out rep involvement.

$(Possiblities.Account Number) = $(Accounts.Account Number)

Example 2
Is it possible to have the first close of an account saved as a field in the account module?

$(Account.First Buy) =
GetDate(Min($(Possiblities.Close Date))