In the source statement of the To IDStore pass apply the case to the LogonTimeStamp column (lastlogon is not synced amongst domain controllers I think).
Example for SQL Server:
select
case when lastlogon = '' then NULL
else dateadd(mi,(cast(lastlogon as BIGINT)/600000000) - 157258080 + DATEDIFF(Minute,GetUTCDate(),GetDate()),0)
end lastLogon
from test_ad_data
Adapted from conversion example I found on the net by "Number2 (John Nelson)"
Br,
Chris